This does not relate directly to your question, but wanted to share the setup I put in place for our PBX.
I left the DHCP server option turned off in the PBX and left it doing ONLY PBX-related tasks. I took two old PCs and installed Lubuntu on them and configured BIND to provide DHCP for all the VLANs in our network, forwarding DHCP requests through to these machines using our edge device. Because I have two machines, I have to change the file on both whenever making changes/additions, but this type of setup has worked perfectly thus far. If either machine dies or is taken down, the other handles all DHCP requests automatically. I also set them up to serve NTP via OpenNTPd and a bunch of other stuff.