Just tested on one and the same PBX:
A: Softphone to softphone (MicroSIP and Zoiper): Almost no audio latency, maybe some 100ms.
B: Softphone to Digium phone (D80): Close to a full second, 1000ms! Someone can say one or two words before the signal arrives on the other end. That kind of latency is not good for the flow of a conversation.
Is there anything one can do about that?