Hi Allan
Your phones typically go into a different VLAN/Subnet to the PCs.
Your DHCP server will most likely be in the PC subnet.
Since DHCP is broadcast based, the broadcast requests for a DHCP server will reach from PC to the DHCP server, but will not reach between VLANs (as a VLAN is a broadcast domain).
On the switches or router that routes between the PC and Voice VLANs, you will typically have an 'ip helper-address' configured. This picks up the DHCP broadcast and unicasts it to the DHCP server. There's probably one set up for the IP of the old DHCP server, and you need to add the new server in there. You can have more than one for the migration period if you want..
Aaron
Aaron
Please remember to rate helpful posts to identify useful responses, and mark 'Answered' if appropriate!