yes you can split the 'internal' ports into individual ports, and even combine some into a switch again.
But - you don't need to. You can create a VLAN as a subinterface of 'internal' and connect your PBX to it.
The point is: in order to break up the switch you need to delete all references to it - address objects, policies, DHCP server, VLANs, VIPs, etc. etc. So you don't do this (after configuration is done) if you don't have to.