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.
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.