Calls don’t have Asterisk unique IDs. What has a unique ID is the an instance of an Asterisk channel data structure. For a simple call, one instance is created when the call arrives, and another instance is created by the Dial application.
Certain operations, e…g unparking also create new channel instances and throw away the old one (more accurately, the old one is turned into a zombie and is thrown away by the thread that owns it.
Recent versions of Asterisk do associate one of the unique IDs with the call itself, but this goes under another name. and the rules for determining when to set and change it may not meet your needs.
All Asterisk unique IDs should be treated as opaque, i.e. your dialplan and applications should not make use of the fact that they are, currently, constructed from a time and a serial number.