We have worked with nearly all the various distributions of Asterisk & FreePBX: Elastix; PBX In A Flash (PIAF), AsteriskNow, and Trixbox and of course vanilla asterisk.
Versions 1.6, 1.8 = Rock Solid
11.x = Mostly Supported
Versions 1.4 = Partial Support
The Asterisk 11 is very popular with our customers, approximately 30% of our customers use it. We are very close to classifying this as completely stable.
Version 1.4 uses AMI v1.0, we will gladly work with customers who have this version. The basic features work, such as direct calls to extensions. Some features like ringgroups and queues will need to be tested further.
If you were to ask which we’d recommend for a new install, it would be PBX In A Flash, Purple (Asterisk 1.8). Mainly because this is the version we test the most internally as it’s our company PBX.