I second Junkhackers statement. Typically, people place VoIP and Computer systems on separate networks. This serves two purposes.
It ensures traffic for VoIP is separated and uninterrupted by everyday network traffic.
It ensures somebody doesn’t accidentally get misleading information for troubleshooting within the network.
Unless your VoIP phones are being handed the exact same IP range as the systems (Highly recommend against) Windows DHCP Scope options can be specified per scope, not per server. This means, if you have 3 networks being handled by the same DHCP server, you can have each network find and get their relevant information all separate from one another.
For example,
192.168.1.X is looking to get Option 66/67 from 192.168.1.5 and undionly.kpxe
172.16.X.X is looking to get Option 66/67 from 172.16.15.1 and pxelinux.0
10.X.X.X is looking to get Option 66/67 from 10.2.5.3 and receives the VoIP phone tftp files.
Again, that’s just a simple break down, but this can all be done from a singular DHCP server.