International calls - playing back my conversations

[#1]

MY wifes grandparents who are Plus Net customers have just rung up all panicked.  They have just had a call that came up on the display as INTERNATIONAL.

When grandma answered it, all she could hear was a conversation she had with her sister (who lives 2 miles from her) a few days earlier.

How is this possible?  They are very bright people and not easily confused so if they say this happened I believe them 100%

Anyone got any ideas?

[#2]

That would be a call for for the police, phone tapping springs to mind

[#3]

This is what I am wondering but I want to give Plus Net a chance to guide us what to do?  Also wondered if anyone else has had the same issue?

[#4]

I've never heard of this before and we certainly have no equipment that is cable of doing this. Definitely one for the police imo.

[#5]

Cheers Ben, it looks like it is a case of monitor it for now and if it happens again then it can be taken further.  But at least the people who need to know are now aware.
Very odd thing to happen though.  It may be worth other people keeping an eye out for anything similar.