> you can have two standards compliant SIP implementations that can't interop I spent years in telco. You would not believe how painful getting telco A to talk

> you can have two standards compliant SIP implementations that can't interop I spent years in telco. You would not believe how painful getting telco A to talk to telco B was, despite it all being SIP. There was a bazillion-dollar market for session border controllers that did nothing but protocol fixup all day. Incredibly, one of the things that SBCs had to do was transcode audio in near-real-time between one telco and another. This was very technically demanding since end users don't like glitchy audio, but also, in some sense, totally a waste of everyone's time and expensive DSP resources, since the two endpoints spoke the same codec but had to cross internal networks that did not.