Most free SIP trunks are trial credit, such as Twilio's 75 free voice minutes or IDT's $25 credit, or a free trunk with paid numbers and minutes. A working trunk for AI calls still needs a number, an emergency address and enough calls at once. Dograh, the open-source AI voice agent platform we build, runs self-hosted or on Dograh Cloud.
Key Takeaways
- Twilio's trial gives 75 voice minutes for 30 days, verified numbers only.
- A "$0 per channel" trunk still bills every number and every minute.
- Every number needs an emergency address before real callers use it.
This post is part of our guide to SIP trunk providers, where free offers sit beside paid rates in the cost section. Below, we sort free trunks into trial credit, free trunks with paid minutes and free inbound, then list what a working trunk for AI calls still needs.
Three kinds of free trunk
A free SIP trunk is almost always one of three things, and only one of them lets real callers reach your agent for nothing. The first is trial credit, a small balance or a set of free minutes that runs out or expires. The second is a free trunk where the connection costs nothing but numbers and minutes are billed. The third is a plan where receiving calls is free and dialling out is not.
None of the three is a free phone line in the way the word suggests. Each gives you something real for testing, and each has a point where the bill starts. Knowing which kind you have tells you what you can test and when to budget.
Trial credit
Trial credit is the most common free offer, and it suits a first test call. Twilio's free trial guide gives product-specific free units, including 75 voice minutes, and the trial expires after 30 days. A trial account can call or message only verified numbers, up to 5 of them, so an outbound agent cannot reach real leads until you upgrade.
Upgrading means adding a business profile, an address, a starting balance and payment details. Twilio's SIP trunking page advertises a free trial with no credit card, so you can try a number and a call before you pay anything.
IDT Express's AI trunk page offers $25 of free credit with no card and no minimum spend, against its own wholesale rates. Plivo mentions trial credits on its SIP trunking page as well. In each case, the credit pays for real minutes at the carrier's normal rates, so treat it as a test budget, not a free line.
A free trunk with paid minutes
Some carriers charge nothing for the trunk itself and bill everything that runs over it. IPComms' trunk testing guide sells SIP trunking at $0 per channel with no contract or setup fee, but a local number costs $1.50 a month, outbound calls $0.010 a minute and inbound $0.009. Its free test account allows 2 calls at once.
The big carriers work the same way under other names. Telnyx's SIP trunking prices list a $0 platform fee and free call concurrency, while every minute and number is billed. Twilio's Elastic SIP trunk includes unlimited concurrency, and its minutes and numbers are billed too.
This kind of free is the most useful one for an AI agent. You pay only for what runs, there is no seat or channel fee to size, and an outbound campaign can start many calls at once without a bigger plan.
Free inbound offers
The third kind charges nothing to receive calls and bills you when you dial out. It can suit an agent that only answers, such as a receptionist, as long as the number itself is free too. Before you rely on one, read the plan for whether the number has a monthly fee, whether the free minutes have a cap, how many calls can come in at once and whether emergency calling is included.
These offers are about the phone line, not the agent. Free AI answering apps are a different product with their own limits, covered in our post on free AI answering services.
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What a working trunk for AI calls still needs
A trunk that takes real AI calls needs five things that no free offer removes. The first is a number the carrier owns. Dograh sells no numbers or minutes, and on Dograh Managed SIP every outbound call needs a caller ID from the trunk's own numbers, or it is rejected.
The second is an emergency address for every number. Under the FCC's 911 rules, an internet phone provider must send a 911 call with the caller's location, and Twilio charges $75 for a 911 call from a number with no registered address. Register addresses before real callers use the line.
The third is enough calls at once. A test account that allows 2 is fine for testing, but a campaign or a busy morning needs more. Self-hosted Dograh runs 10 calls at once by default, a limit you can raise, and Dograh Cloud pay as you go includes 10. The fourth is encryption, TLS and SRTP, which Twilio, Telnyx and Plivo include on their trunks at no charge. If a Twilio trunk carries the agent's outbound calls from Managed SIP, leave its Secure Trunking setting off, since that leg is not set up for TLS.
The fifth is the platform and the models. Managed SIP itself has nothing to pay on the Cloudonix side, but no calls flow until a carrier is connected. Self-hosted Dograh has no per-minute platform fee, but servers, model keys and phone minutes still cost money. Dograh Cloud charges a 1¢-a-minute platform fee.
The model line depends on what you pick. With your own keys, each provider bills at its own rates, or open-weight models on your own servers turn it into a GPU bill. Our breakdown of what an AI voice agent costs per minute puts a number on every line of the bill.
Testing an AI agent on a free trunk
A free trunk is good for one job, which is proving the agent works on a real phone line before you pay for volume. Use it for that and plan the move to a paid plan from day one.
Call the agent from a mobile and from a landline, and listen on the handset, since most phone calls reach the agent as 8 kHz audio while a browser test runs at 16 kHz. Run two calls at once to check the trunk's limit. Place an outbound call to confirm the caller ID shows your number. If the agent hands callers to staff, transfer one test call to a colleague.
When the test passes, price the real month. Our post on SIP trunk pricing works through a full month of AI calls on three carriers, fees included.
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Glossary
- Free units
- A set amount of usage a trial includes, such as Twilio's 75 voice minutes. They expire with the trial, after 30 days on Twilio, and any usage beyond them needs an upgraded account.
- Verified number
- A phone number you have confirmed you own. A Twilio trial account can call or message only verified numbers, up to 5, so an outbound AI agent cannot reach real leads on a trial.
- Caller ID
- The number shown to the person an outbound call reaches. On Dograh Managed SIP it must be one of the trunk's own numbers, which the carrier owns and bills monthly, or the call is rejected.
- Emergency address
- The location on file that a carrier passes along when a number dials 911. Free offers rarely include it, and Twilio charges $75 for a 911 call from a number without one.
