Many voice communications products implement SIP-PSTN signaling interlocking, which includes SIP source code software, an ISDN protocol stack, and an T1 RBS E1 R2 CAS protocol stack
SIP-PSTN Signaling Interworking Features
The small RAM and ROM footprint of a quality embedded software development toolkit means that you can save memory. And the fast execution speed allows use of a slower processor; fewer instruction cycles means that the processor can "sleep" more thereby lowering power requirements.
These devices may VoIP-enable a standard POTS or ISDN telephone, or may allow a local VoIP phone or softphone to access the local PSTN. Even if a customer has a VoIP phone connected to a VoIP phone provider, they may desire some calls to go to the local PSTN because of quality or network congestion issues (i.e. "best quality routing"), and the customers may desire that all emergency "911" type calls go via PSTN instead of VoIP.
The low per-call memory requirement of a quality SIP stack means that you can save a significant amount of memory when your gateway services thousands and tens of thousands of calls. And the fast execution time results in more calls per MIPS, or in lower power requirements for a given number of calls. This translates into lower power requirements for your system, less heat generated by the processor, lower air conditioning requirements--in short, better NEBS performance.
These devices may VoIP-enable a standard PBX, or allow a network of VoIP phones and VoIP softphones connected to an IP PBX or router to interwork calls to the PSTN. Even if a customer has an internal VoIP network connected to a VoIP service provider, they may desire some calls to go to the local PSTN because of quality or network congestion issues ("best quality routing") or for cost reasons ("lease cost routing" and "toll bypass"). Also, customers may desire that all emergency "911" type calls go via a local PSTN rather than their VoIP service provider.