I would recommend using Pocket PC
2003 version normally. WM 5 or 6 may have significant issues with phone
compatibility. If you can go back to 2003 and the phone still doesn't work there may be a hardware issue, then you need to replace or repair. Replace is usually the better option. Search ebay.com if that is what you need.
Below, instrutctions on getting ROM for PPC 2003:
and ebay for ROM upgrades. There are many people who unlock these kinds of devices. I don't believe you can upgrade it to WM 5.0 or beyond, but that may be possible. Search for WM versions that include "unlocked" versions. The best bet is a newer version of PPC 2003.
The process is relatively simple. Once you have your ROM on CD or via download, You run it with your device hooked up to your PC
/cradle and connected via ActiveSync (3.7 or later). The warning is an improper ROM load could make your device unusable. It is a risk you have to take if you want it to make calls. Toss up: no phone Pocket PC, pocket PC phone, or shiny brick. If the XDA site has recommendations on a ROM source, they are not likely to steer you wrong. They will state emphatically, that you do not want any interruptions in the ROM upgrade process. If you have a power outage or something shuts off for any reason, you will have the shiny brick and be looking for a new device.