Another answer phone problem

Problem has been occurring for the last 5 or 6 weeks.

It appears that when I have several messages I go to listen to them and part way through the playback the line goes quite - it does not disconnect me from the line, just the message goes silent.  You cannot delete it at this time, all you can do is hang up and dial the 1571 again to connect and then delete the message as usual.  This then moves on to the next message which sometimes does exactly the same or sometimes doesn't?? - there is no rhyme or reason as to when it will do it, only that the problem will occur at some stage during listening to my messages.

I sometimes have to call back 5 or 6 times just to listen and delete my messages - help required please.