It is tempting to look at a soft PBX as a one-stop-shop for VoIP, with no other product required. In fact, this is a very limited strategy in terms of features and resilience.

Many people have a requirement to use existing ISDN lines with their RTC architecture. With Asterisk and FreeSWITCH both boasting ISDN support, it is tempting to do everything with that single instance of a soft PBX installed on a server with ISDN hardware.

Modularity is a hallmark of good design in any IT project. Modularity gives you the flexibility to upgrade or modify one component of a system without changing any other component. Modularity also means that some components are more likely to continue providing service even if something fails. In the planning of a soft PBX deployment, modularity involves running one instance of the soft PBX just to control the ISDN hardware and running another instance for services such as voicemail and using a SIP proxy to route calls between these different components.