We built this because nobody else did.
The story of shipping enterprise telephony integrations the hard way, and finally building the tool that makes it easy.
Every integration, the same problem
We launched Call Sandwich to build Retell AI integrations for enterprise clients. From day one, the work was exciting and the results were real. AI phone agents were genuinely useful, and clients wanted them fast. But there was always the same friction point waiting for us.
Most enterprises already have a PBX
Asterisk, 3CX, hosted VoIP systems, legacy on-prem installs. These are registration-based SIP systems, meaning phones and trunks register to the PBX server. Retell AI is different: it uses outbound SIP URI dialing. Bridging between these two worlds is not natively supported. Something in the middle has to translate.
We tried every approach. None were easy.
Enterprise SBCs from vendors like AudioCodes or Ribbon solve the protocol gap, but they start at $10,000 per year, require certified engineers to configure, and add a permanent maintenance dependency. Not right for most clients.
FreeSWITCH is incredibly capable, but configuring it as a SIP bridge is not a weekend project. It requires deep telephony engineering, meticulous SIP routing configuration, and ongoing expertise to maintain. Every deployment was weeks of work.
Sometimes we would push clients to rebuild their telephony stack. This was the most complete solution but also the most disruptive. Months of work. High risk. And most enterprises have legitimate reasons to keep their existing infrastructure.
The problem was always the same. The solution should be too.
Across dozens of deployments, we kept solving the same puzzle. The gap between registration-based PBX and outbound SIP was a known, repeatable problem with a known, repeatable solution. It just needed to be productized. So we built it.
sipsandwich.ai
A self-service SIP switch that any team can set up in minutes. Register your PBX, map your extensions to Retell AI agents, and go live. The engineering complexity stays on our side. Your team gets a working AI phone system.
Three steps. That is it.
Register your PBX
Point your PBX SIP registration at sipsandwich. Works with any registration-based system: Asterisk, 3CX, hosted PBX, or custom SIP infrastructure.
Map to Retell agents
In the sipsandwich dashboard, assign your DIDs and extensions to specific Retell AI agents. Inbound calls, outbound campaigns, specific queues — configure routing exactly how you need it.
Go live
Calls route seamlessly between your existing PBX infrastructure and Retell AI agents. Your phones stay, your numbers stay, and your agents start handling calls immediately.
The engineering hell, handled on our end
sipsandwich abstracts the SIP session translation, registration management, and call routing complexity so your team does not have to touch any of it.
Try it today.
Self-service signup, no sales call required. Your PBX can be routing calls through Retell AI before end of day.