Siemens SL55 Phone problem.

[TEAK36]

I have just purchased a new Siemens SL55 phone,but if I try and ring it using my landline, I get an engaged signal.I have set everything else up ok and can send and recieve SMS.The WAP works fine, and I can make outgoing calls.The phone was Sim free when I bought it and I am using my pay as you go T Mobile Sim card which I have been using for about 5 years.I was thinking maybe I have something set up wrong, but I have read the manual several times and can find no reason why this is happening.Any help would be most appreciated.