It is not best practices to have more than one DHCP server. This role is frequently handled by a router, a firewall, a servev or sometimes a layer three switch. You say that your phone system is handing out DHCP, and that may be necessary if your phone system is a VOIP phone system. In this case, the best thing might be to put all of you phones, and phone system on one VLAN on your switch, and your PC's on a default VLAN, or a VLAN just for computers. You will have to have a manageable switch, such as a Cisco, 3Com, or HP Procurve in order to do the vlan (They all have models of managable and non-managed switches. The Vlan seperation may be better anyway in order to do Quality of Service (QOS) on your VOIP system later.
If you do not have any VOIP phones, you could disable DHCP on the phone system.
If you have a Windows network, such as Windwos 2000, or Windows 2003 or Windows 2008 server then that server should be your CNS and DHCP for your network. If not, let me know what type of servers you have and I can suggest an allternative.
