Here's the catch. "Unlimited" almost always means unlimited domestic minutes under a defined plan — not unlimited simultaneous calls, unrestricted international dialing, or unlimited call-center traffic. Read the fine print before you sign anything.
This guide compares leading US providers across pricing structure, concurrent-call capacity, PBX compatibility, reliability, support, compliance, and fair-use terms. By the end, you'll know what questions to ask before you switch.
Key Takeaways
- Unlimited SIP trunks deliver flat-rate PBX-to-PSTN calling under the provider's usage policy.
- Size by concurrent channels, not headcount; channels set how many calls can run at once.
- Fair-use policies commonly restrict autodialers, predictive dialing, telemarketing, and call-center traffic.
- Build the full monthly cost: channels, numbers, E911, taxes, setup, porting, and international rates.
Overview of Unlimited SIP Trunking in the US Market
SIP trunking connects an on-premises, hosted, or hybrid PBX to the public switched telephone network (PSTN) over an internet connection. No PRI circuits, no copper lines — just SIP signaling running over your existing broadband.
Businesses switch for a practical reason: fewer circuits to manage and a lower fixed monthly cost than PRI.
Trunks, Channels, and Concurrent Calls: Know the Difference
These terms get used loosely, but they matter for sizing your plan correctly:
- Trunk — the SIP connection itself, linking your PBX to the provider's network
- Channel — the capacity unit for one simultaneous call (most providers size one channel per concurrent call as a baseline)
- Concurrent call — a call actively in progress right now, counted against your channel limit
- User — an employee or extension, which has no fixed relationship to channel count
- Minute — a unit of talk time, relevant mostly for metered plans
A 50-person office rarely needs 50 channels. What matters is peak simultaneous call volume, not headcount. A receptionist desk with two lines gets far more traffic than a back-office team of 20 who each take one call a day.
When Unlimited Makes Sense — And When It Doesn't
Unlimited plans suit offices with steady, predictable domestic calling: sales teams, clinics, professional services firms. But they're a poor fit for:
- Low-volume businesses paying a flat rate for calls they rarely make
- Seasonal or bursty traffic (retail peaks, holiday spikes) that leaves you overpaying in slow months
- Companies with heavy international calling needs
- Automated dialing or call-center operations
Use that sizing lens when you compare how the major US providers price and cap unlimited trunks.
Top Unlimited SIP Trunk Providers in the US
The following comparison is based on current published plan pages, service terms, and acceptable-use policies. Providers frequently update pricing and policy terms, so confirm details directly before committing.
SIP.US
SIP.US publishes one of the clearest flat-rate benchmarks in the market: $24.95 per channel per month, with unlimited outbound calling to the US48 states and Canada, plus unlimited inbound calling to local numbers, according to SIP.US's pricing page. There's no contract and no commitment required.
The platform supports a wide range of SIP-enabled PBX systems, including 3CX, Asterisk, Elastix, Cisco, Avaya, and Grandstream, along with open-source telephony applications. That breadth makes it a practical fit for businesses that already have PBX hardware in place.
Additional capabilities include:
- US48 number porting through a self-service Control Panel
- Toll-free DID availability
- Business SMS, SMS/MMS through partner apps or APIs
- Fax-to-email and email-to-fax support
- Support via ticket system, knowledge base, email, and phone
Comparison snapshot:
- Pricing: $24.95 per channel/month, no contract
- Included calling: Unlimited outbound US48/Canada, unlimited inbound to local numbers
- Channel rules: No published maximum; one channel supports one concurrent call
- Fair-use limits: Prohibits autodialers, predictive dialers, robocalls, telemarketing, call-center operations, and fax broadcasting
Nextiva
Nextiva's SIP trunking connects to an existing local or enterprise PBX rather than requiring a full hosted phone system swap. The published pricing shows metered service at $14.95 per month plus $0.008 per minute, or unmetered service at $24.95 per month with unlimited nationwide calls and no per-minute charges.
Capacity works the same way SIP.US describes it: one trunk line handles one call at a time, so 20 simultaneous calls require 20 trunk lines. Compatible PBX systems include Avaya, Asterisk, Cisco, 3CX, FreePBX, and Mitel.
Nextiva states that setup, support, and number porting come at no additional charge. Additional DIDs, toll-free numbers, and extra call paths are quote items rather than published flat fees.
Comparison snapshot:
- Pricing structure: $24.95/month unmetered or $14.95 + per-minute metered
- Included domestic calling: Unlimited nationwide on the unmetered tier
- Requirements: Existing PBX plus local IT administrator for setup
- Acceptable-use terms: Not explicitly published in the reviewed trunking FAQ — confirm in writing before deploying autodialers
8x8
8x8's official pricing pages point businesses toward custom communication plans and bundled packages rather than a standalone, publicly priced SIP trunk. That's an important distinction if you're specifically shopping for pure trunking rather than a full UCaaS platform.
No public standalone SIP price, included-destination list, channel capacity figure, or fair-use language was available in the reviewed pages. If 8x8 is on your shortlist, request a written, product-specific quote that confirms:
- Whether the plan connects to your existing PBX or requires 8x8's hosted system
- Channel or concurrent-call capacity
- Included domestic and international destinations
- Fair-use restrictions on call-center or automated traffic
Comparison snapshot: Treat 8x8 as a UCaaS/contact-center provider first. Pure SIP trunk terms require direct confirmation before comparing against channel-based competitors.
Vonage
Vonage documents SIP trunking designed to link existing PBX infrastructure to the cloud, with configuration guides for common systems. The company states capacity can scale up or down as needed, and trunks can reportedly be provisioned in minutes.
That said, the reviewed page doesn't publish a monthly price, a domestic flat rate, or a numeric channel limit. Global inbound and outbound calling are mentioned, but unlimited domestic calling isn't explicitly established in the public materials.
Comparison snapshot:
- Pricing: Quote-only; no published rate
- Calling scope: Global inbound, outbound anywhere with Vonage local numbers
- Capacity: Scalable, no published channel cap
- Fair-use terms: Not clearly stated in reviewed policy — get written confirmation for autodialing or high-volume use
CallHippo
CallHippo's pricing page describes a per-user office phone system rather than a channel-based SIP trunk schedule. Bronze and Silver land around $16 and $23 per user/month, with unlimited manual-dialing minutes to standard US/Canada numbers. Enterprise, at about $39 per user/month, extends unlimited calling to 48 countries.
Unlimited calling excludes premium and toll-free numbers, and local taxes apply on top of listed rates. CallHippo's own content describes the company as a SIP-trunk provider offering cloud PBX, but a separate, channel-based SIP pricing schedule wasn't confirmed in the reviewed materials.
Comparison snapshot:
- Pricing model: Per-user, not per-channel
- Included calling: Unlimited manual dialing to US/Canada (Bronze/Silver); 48 countries (Enterprise)
- Exclusions: Premium and toll-free numbers, plus local taxes
- Best fit: Sales teams wanting click-to-call and dialing tools bundled with the phone system
Public Telephone Company lists SIP trunking alongside UCaaS and hosted VoIP, with scalable deployments for multi-location and existing-PBX environments. Like other quote-led providers above, channel rates and unlimited-plan terms aren't published for a side-by-side rate card—request written trunk terms, fair-use rules, and concurrent-call capacity before you compare.
How We Chose the Best Unlimited SIP Trunk Providers
We verified current plan pages, service terms, acceptable-use policies, and PBX compatibility guides rather than relying on advertised headline prices alone. Here's the framework:
Total cost of ownership. We looked at base charges, channel or user fees, DID numbers, E911 charges, taxes, porting fees, setup costs, international rates, minimum commitments, and regulatory recovery fees, not just the sticker price.
Technical performance and scalability, including:
- Concurrent-call capacity and bursting allowances
- Bandwidth requirements and codec support
- Failover routing and redundancy
- Published uptime commitments
Operational fit. Number portability, caller ID handling, call routing, voicemail, analytics, CRM/UC integrations, and compatibility with the PBX you already own or plan to deploy.
Fair-use and compliance terms. These carried real weight in this comparison. We checked restrictions on autodialers, predictive dialing, telemarketing, call-center traffic, and short-duration calls.
We also confirmed baseline regulatory context: the FCC requires voice service providers to implement STIR/SHAKEN caller-ID authentication across IP voice networks, and interconnected VoIP providers using the PSTN must meet E911 obligations. Any provider that's vague about either topic deserves a follow-up question before you sign.
Conclusion
The cheapest advertised rate isn't automatically the right answer. Your actual choice depends on call concurrency, traffic type, your existing PBX environment, calling destinations, support expectations, and the fully loaded monthly cost once every fee is on the table.
Before signing with any provider:
- Request a written quote and sample invoice, not just a headline price
- Confirm the fair-use policy in writing, especially if you run any automated or high-volume dialing
- Test call quality on your own network before committing
- Validate E911 and number-porting procedures with specifics, not assurances
- Ask how they handle outages or channel exhaustion during peak demand
If you're weighing a broader communications overhaul rather than trunking alone, Public Telephone Company offers scalable SIP trunking, PBX, and UCaaS with cloud-hosted deployment options and 24/7 support. Systems scale from small teams to 100,000+ users.
Pricing and plan specifics vary by deployment, so a tailored consultation is the right next step rather than assuming unlimited terms apply universally.
Frequently Asked Questions
How much does an unlimited SIP trunk cost?
Pricing varies by provider and may be charged per channel, per trunk, per user, or as a bundled plan. Published US benchmarks run around $24.95 per channel/month, plus separate costs for numbers, E911, taxes, and setup.
How many concurrent calls can an unlimited SIP trunk handle?
Unlimited minutes don't mean unlimited simultaneous calls. Each channel typically supports one concurrent call, so capacity depends on how many channels you've purchased, not your calling volume.
What is an unlimited SIP trunk in simple terms?
It's a flat-rate, internet-based connection between your PBX and the phone network that includes a defined scope of calling. Plan terms still govern which destinations, traffic types, and volumes are actually covered.
Are unlimited SIP trunks really unlimited?
They cover normal business calling within specified destinations. Most providers restrict or exclude autodialers, predictive dialing, telemarketing, call centers, and abnormal usage patterns under fair-use policies.
Is an unlimited SIP trunk better than metered SIP trunking?
Unlimited plans suit steady domestic office traffic where flat billing beats tracking per-minute charges. Metered plans often make more sense for low-volume, seasonal, or heavily international calling patterns.
Do unlimited SIP trunks support call centers and autodialers?
Many unlimited plans restrict or prohibit predictive dialing, autodialing, and high-volume call-center traffic. Get written confirmation from the provider, or choose a plan priced for that use case.
