Direct answer
SIP trunking connects a phone system you run to the public telephone network. A cloud PBX is the phone system itself, run by a provider on its own servers, with that connection built in. So the choice is about who operates the PBX. You either keep one and buy trunks for it, or hand call control to a provider and pay per user.
With SIP trunking, your desk phones and apps register to a PBX that you control. It can sit in your office, in your data center or with a hosting company. Twilio's trunking documentation lists the equipment you supply on your side: a session border controller, a SIP call server or an IP-PBX with internet access. That equipment connects to the carrier over the public internet or a private link, and the carrier completes the call on the public network. Your PBX also has to prove the traffic is yours. Twilio requires an IP access list or credentials at a minimum, and it recommends credentials.
With a cloud PBX, the phones and apps register over the internet to the provider's servers, and there is no call server on your side. The office keeps its phones, its network and its internet connection. What you get is a working phone system. Atlantech's hosted service, for one, lists auto attendants, ACD groups, a receptionist console and voicemail transcription, and its softphone runs on Windows, Mac, tablets, iOS and Android.
The trunk model leaves compatibility with you. The carrier, the IP-PBX and any border device between them have to work together, and Atlantech's compatibility guide says mixed-vendor equipment can need software upgrades or gateways before it does. The guide advises involving the carrier early, which in practice means sending your PBX model and software version before you buy. A cloud PBX takes the core PBX off that list. The phones still have to work with the provider's platform, so Atlantech supports most SIP-compliant phones and recommends devices from its Approved Endpoint List.
Trunks are billed by the concurrent call or by the minute, depending on the carrier. SIP.US charges $24.95 per channel per month on a month-to-month plan, or $19.99 on its yearly plan, with calls to the lower 48 states and Canada included. A channel is one call in progress, so an office buys as many channels as its busiest moment requires. Twilio charges by the minute with unlimited concurrent calls. Its rates are $0.0100 a minute for outbound calls at the 48 States rate and $0.0034 a minute for inbound calls on local numbers.
A cloud PBX is billed per user. Atlantech lists a Business Line starting at $12 per user per month and a Premium Line starting at $18. The tier matters, because voicemail, conferencing, collaboration tools and the softphone come with Premium.
Suppose an office has 20 users, 20 direct numbers and no more than 8 calls in progress at its busiest moment. For the per-minute carrier, assume 6,000 outbound minutes a month at the 48 States rate and 4,000 inbound minutes on local numbers. The figures in the table below are list prices before taxes and fees.
Monthly cost for a 20-person office
| Option | Calculation | Monthly |
|---|---|---|
| SIP.US trunks, month to month | 8 channels x $24.95, plus 20 local numbers x $1.00 | $219.60 |
| SIP.US trunks, yearly plan | 8 channels x $19.99, plus 20 local numbers x $1.00 | $179.92 |
| SIP.US trunks, month to month, with nomadic E911 numbers | 8 channels x $24.95, plus 20 enhanced numbers x $2.50 | $249.60 |
| Twilio trunks, per minute | 6,000 outbound minutes x $0.0100, plus 4,000 inbound minutes x $0.0034, plus 20 numbers x $1.15, plus emergency calling at 20 x $0.75 | $111.60 |
| Atlantech Business Line seats | 20 users x $12, the starting price | $240.00 or more |
| Atlantech Premium Line seats | 20 users x $18, the starting price | $360.00 or more |
The trunk rows and the seat rows buy different things. The trunk figures cover call paths and phone numbers. The PBX behind them comes on top: its licenses, its support contract and the hours someone spends patching and securing it. The seat figures are starting prices for the whole phone system, and the $6 difference between Atlantech's two tiers buys voicemail, conferencing, collaboration and a softphone for each user.
Per-channel trunks follow peak calls. The same office with 4 peak calls would pay $119.80 month to month, and with 16 it would pay $419.20. Per-minute trunks follow usage, so doubling the assumed minutes adds $73.60. Seats follow headcount, at $12 or $18 for each new hire.
Calls in both models use the office's connection to the provider, which is the internet circuit unless a trunk runs over a private link. Twilio sizes its trunks at 100 kbps per simultaneous call, so 8 calls need 800 kbps set aside for voice.
When one office loses power or internet, a cloud PBX sits outside the affected building, so that outage doesn't stop the phone system. The desk phones in that office stay offline until the power or connection returns. Atlantech notes that a hosted PBX stays up through an on-site power loss and that the mobile softphone can serve as the backup.
A PBX in that office loses power with the building unless it has backup power. If only the internet fails, the PBX and internal calls keep working, but it can't reach the carrier without a second path. The carrier then has to send calls somewhere else, and you have to tell it where ahead of time. Twilio lets you set a Disaster Recovery URL that takes over when your PBX can't be reached. Another provider, 2talk, advises diverting main numbers to cell phones, a group or an auto attendant, moving users to a softphone app, and recording SIP credentials and device details before anything fails.
When the PBX itself breaks, the repair is yours on a trunk and the provider's on a cloud PBX. The carrier runs redundant entry points, and setting your PBX to use more than one of them is your job. Twilio's edges each resolve to three or four IP addresses, and its guidance is to try the next address after four seconds and to avoid pointing the PBX at a single one.
With a cloud PBX, remote staff sign in to the app from a home or mobile connection and keep the same line. With trunks, the PBX has to reach remote staff through its own softphone or remote extensions, which you publish and secure. Atlantech's comparison gives one version of that: mapping a home phone to the work line.
Call quality on the office circuit is your job in both models. Trunks need a bandwidth analysis against peak calls, and a hosted phone depends on a stable connection at every site.
Start with the PBX you have. If it is under vendor support and a named person patches it, trunks let you keep using it. If it is near the end of support or nobody maintains it, a cloud PBX hands that work to the provider.
Then compare peak calls with headcount. In the example above, 8 peak calls across 20 users put three of the four trunk totals below both seat prices, before any PBX costs. That gap closes in an office where most people are on the phone at once.
Look at how remote staff reach the system today. If the PBX already serves them through its own softphone or remote extensions, trunks change nothing for them. If they forward calls to cell phones, a cloud seat with an app is the shorter path.
Count the admin hours you have each month, because trunks leave the dial plan, the security settings and the failover plan with you. Then total both sides. Add PBX licenses, support and endpoints to the trunk quote, and add any tier upgrade to the seat quote.
Picking a model doesn't have to mean picking a different carrier. Atlantech's business phone page says customers can connect to its cloud PBX or, with their own PBX, take service over SIP, PRI or analog lines. Its UCaaS page also describes SIP trunking that extends its softphone to users on an existing PBX. That gives a business a staged route: add softphones for remote staff now and replace the hardware later. The route depends on the PBX working with Atlantech's voice switch, so test that before you sign. Atlantech publishes starting prices for hosted seats and quotes its trunk pricing.
Either model needs a plan for the location sent with a 911 call, for desk phones and for remote users, and what Enhanced 911 requires covers that. A support team that handles calls and chat should look at unified queues for phone and web chat alongside the phone-system choice.