Add ip address 192.168.2.x/24 to LAN interface (bridge) takes care of routing between two subnets on same interface.
add address=192.168.2.254/24 interface=bridge network=192.168.2.0
Extend ip pool and add dhcp-network for this segment.
/ip dhcp-server network
add address=192.168.2.0/24 dns-server=192.168.2.254 gateway=192.168.2.254
add name=lan2 ranges=192.168.2.10-192.168.2.250
set <num> next-pool=lan2
If supported by the phones, you could use vendor class (instead of next-pool) to assign ip from new pool.
/ip dhcp-server vendor-class-id
add address-pool=lan2 name=lan2 server=default vid=<identifier>
Bridge is already member of interface list LAN so firewall should be ok.
No NAT is required, PBX should allow connections from 192.168.2.0/24.