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Unicall is an abstraction layer, making telephony applications largely independent of the protocols used to signal and transport phone calls. It consists of a very simple library, which is called by the telephony applications, and into which telephony protocols are plugged. Unicall is similar in nature to things like GlobalCall from Dialogic. If are familiar with using GlobalCall, Unicall should look quite familiar.
Unicall is based around the call model used in ISDN and SS7. This is essentially a superset of the call models used by other telephony protocols. That means they usually plug into the Unicall framework without too much hassle.
A large (some would say ludicrous) number of signalling protocols have existed over the life of the public telephone network, and across the world's administrations. The features they offer, and the call model on which they are based, varies considerably. However, these days everything has to be squeezable through an SS7 or ISDN channel. The call model used by these modern protocols is, essentially, a superset of all other call models. Basing an API around the ISDN call model, with a suitably flexible attitude, can lead to an API which will serve any existing form of telephony signalling. Does VoIP change this? Not really. The VoIP signalling protocols are based on the ISDN call model too.