I think whoever told you should give more details. With the right equipment one might be able to tell you had a mobile call. I do not know if digital mobile security is still secure enough to avoid tapping. It's possible control signals could be decoded. But landline needs something in the line to monitor it (or access to your account on BT or whoever). It seems rather unlikely to me, and maybe someone misunderstood what could be done, and we have a case of "Chinese Whispers".