Also, try the basic stuff like transfer/conference. Check numbering format on your display and when using IP phones, check your call log for correct entries and being able to use callback from the log. Test the internal phonebook of the IP phones and check if backup/restore works on them as well. You'll need that for flex working.
Some other stuff you might wanna check is simply unplug a phone and replug it. Sometimes during installation stuff on the network is changed for tweaking, which might cause problems that can occur later on (dhcp server, lldp, routing).