My wife has an o2 contract with copious free minutes and texts but
she's just been charged for two calls she's allegedly made to a number
beginning with 07820.

The number isn't in her contacts so she's adamant she didn't ring it
although she does vaguely remember getting a message telling her she
had voicemails and when she tried to retrieve them she got a "the
person you are calling is engaged" recording.

Anyone shed any light on why these calls might have been charged?
It's only 23p so it'll cost more to query but I'm puzzled.