A couple of days ago, missus had a missed call on her mobile, which
left caller ID and she called it back. It rang for a while, then
connected to an answerphone.

Yesterday, I had the same. Both our numbers are very similar (only
last 4 digits are different) - though one is O2 the other T-Mobile.

It would appear the number is not included in the free minute bundle
and chargeable. O2 said it was a premium rate number - so I assume
this is some arsehole scam to get you to ring a premium rate number
for no reason whatsoever - and effectively pay them for the
priviledge!