Strange intermittent problem.
 

For the past few weeks we have been seeing a strange issue with our land line. When we get a call from a mobile phone we are unable to hear the other person talking but they seem to be able to hear us. At first i thought it was our handset as it was about five years old but even after replacing it the problem still shows up every now and then.
I've tested using two mobiles each on different networks but the issue still remains. Now here is why i think it might be a Virgin problem. This started the same day as a tech came out to check the line for a unrelated problem. Could he/she have done anything in the box outside that could have caused this ?