This happened to me a couple of years ago with a phone I had, it was a BT Decor 110. I learned that sometimes when I thought I wasn't getting through, I was, but the person on the other end couldn't hear me. Anyway, I'd only had it 6 months so I took ti back to Argos and they gave me a replacement, which was the same model number but seemed slightly redesigned. Sounds to me just like a problem with the phone, not the line, but if you are with BT, they will also be able to test the line out for you for free. Hope this helps!