if one person in the marriage is unhappy about what the other is doing to get sexual thrills, then it needs discussing, and stopping as necessary.
i think phone lines are a step up from watching porn... which is something you can do together and get your thrills as a couple, whereas a furtive phone call is not good for both of you.
i;d be mightily p*ssed off, especially if this was something previously discussed and he had promised not to do it again
what is more important , his marriage or some bored lady in her tatty dresssing gown, pretending to be a busty temptress?