DHCP or static IP addresses?
If static, make sure you have your phones IP address, MAC address, model, TN and DN very well documented and the document itself subject to good change management (in other words - don't let the entire IT dept have their hands stirring that pot of soup).
If DHCP, make sure you don't have more than one DHCP server on your network and that the vendor-specific options are set correctly.
The network itself... are switchports configured correctly? Proper voice and data VLAN, etc...? Are the phones themselves configured correctly? IP address, subnet and gateway correct? Right signalling server IP's and TCP port?
Are phones being moved from one VLAN to another on the network - and not changing the VLAN in the phone's configuration?
Start out by printing out a list in LD 117 (stat iset all) and see what's registered and offline to give you an idea of what's what.
Nortel IP phones aren't as plug-and-play as Cisco's unless you're using a provisioning server and DHCP. But if you're using static IP addresses - they'll require a degree of asset management.