The CAMEL Phase 3 & 4 have certain features, which were not available in CAP 1 & 2. The CAP 3 includes all featues of CAP1 and CAP2 plus additional features for Data charging. Other than this, there are certain other messages also, which were added in CAP 3, which could be useful from the services point of view.
CAP 4 includes some additional features like MT SMS triggereing. The main feature of CAP 4 is ICA (Initiate CAll Attempt), which was not there in old releases.
Through this feature, the SCF can initiate the dialougue on it’s own, without any triggering from the SSF.