Wahey!
Finally got the issue resolved! It turns out it was an account configuration problem: call barring level 6 or something I think the customer services rep said!
Anyhow that's been removed & I can make and receive calls! As to not being able to receive them in the first place, I believe I'd placed call forwarding to my mobile on the line prior to it going funny!
Anyhow if you experience similar problems to those I mentioned in first-post, then kindly decline an engineer visit until they've scoped your account out!
Thanks 4 the responses received
MJ