Two DHCP servers on my network

Hi

My Windows 2008 R2 system is configured on the 192.168.1 subnet and DHCP is offering up valid IP addresses to clients fine.

The end user has an IP Phone system so I decided to use the external IP interface on the telephone handsets to wire up my Windows clients. I connected the telephone system switch to the Windows switch and off we went.

First problem is that the telephone system has its own DHCP server offering IP addresses up on the 192.168.91 subnet.

Any newly connected clients get a 192.168.91 address but if I do an ipconfig /release followed by a /renew they then magically switch over to the 192.168.1 network.

Today the on-site IT support decided to accidentally take down the DHCP server and all clients are now picking up 192.168.91 addresses.

He's now disconnected the connection to the telephone system and is going round connecting directly into the computer network to pull up 192.168.1 addresses again.

I've researched the issue but it doesn't appear possible to elect a preferred DHCP server so has anyone any idea of best practices for this environment as it is handy to use the telephone network?