Trying to troubleshoot Sagem MW3020

[bobcratchet]

Hi guys, I'm a real mobile newbie, so I hope you'll treat me gently!!

I have a Sagem MW3020 which was originally PAYG on T-Mobile. It hasn't been used for a while but, when I power it up with the original SIM, it displays date and time, battery charge and network coverage.
When I dial a number, it appears to be calling OK - display changes, shows number dialled and shows a graphic of a receiver - but doesn't ring (either on the MW3020 or on the target phone), then cuts off.
When I call the MW3020 from another phone, it doesn't ring (either end) and I get a recorded message - "The other person has hung up".

When I swap the SIM for a proven, working Virgin SIM, I get the same results.

Do these symptoms sound at all familiar to anyone? As a self-confessed mobile ignoramus, I've no idea whether they suggest a hardware or software problem so apologies if I've posted in the wrong forum.

Thanks in advance for any help you're able to offer.
Regards,
Bob