Sorry, I must have misunderstood your question.
It is not possible for the Key Pad and the PC to have the same address.
Typical Scenario:
Key Pad requests DHCP
Key Pad Gets IP from Data Lan (e.g.) 10.10.10.2
Key Pad gets DHCP Options (VLAN and Priority)
Key Pad Releases IP
Key Pad Requests DHCP on Voice VLAN
Key Pad gets IP from 3300 (e.g.) 192.168.1.11
Key Pad Registers and upgrades if required and is functional
PC requests DHCP
PC Gets IP from Data Lan (e.g.) 10.10.10.3
Done
There is an island of opportunity in the middle of every difficulty.
Miss that though, and you're pretty much doomed!