Your situtation is ideal, PBXMan, but as Kva implied, often this is not the case with legacy switches.
You could very well find version 3 and 4 switches with analogue trunks rather than a full DID range carried over an E1 link. If you're lucky the circuit IDs and IDDDs may be listed on the telco's termination, although this is not always the case.
In a PBX network, additionally, the DIDs may all travel in to one switch which then in turn passes the call to others. Again more complication.
Often digit conversion is needed on the incoming trunk, potentially being linked to AAR via a FAC. If you have many PBXes and need to setup Speed-Dial lists (using abbreviated dial system lists for example) again digit conversion is needed, but this time on the remote PABXes).
Shame there's no intelligent way to manage multi-site speed-dialing using signalling and single-point adminstration. Ditto for time of day sync (not clock cycles - this is possible).