This phone is like 20 years old, but it still works with your ISP's SIP service is configured correctly. Firstly, it's best to update to the latest available public firmware version, however do to this you'll probably need to factory reset it, and convert from UniStim to SIP firmware (version 2 or 3.2) prior.
Per this guide to factory reset:
You can factory reset a Nortel IP phone that is already running UNIStim firmware release 3.0 or later. You will need to be running the following firmware at a minimum for this procedure to work;
IP 1110 (0623C6E), IP 1120E (0624C6E), IP 1140E (0625C6E), IP 1150E (0627C6E), IP 1210 (062AC6E), IP 1220 (062AC6E), IP 1230 (062AC6E), IP 2001 (0604DCD), IP 2002 (0604DCD), IP 2004 (0604DCD), and IP 2007 (0621C6D).
The UNIStim firmware release 3.0 for IP Phones introduces the ability to restore an IP Phone to a “factory default” configuration.
