i have read all through this and can really sypathise with you daisy.
about 5 years ago, i opened the phone bills to find my dh's bill was nearly all this one number. it was the first number he rang in the morning and the last one he rang at night. it was almost everyday and even when we were away on holiday. i asked him who it was and he just said 'oh it's sarah's number'!!
i flew off the handle, but he couldnt see what he had done wrong. he reconed everything was perfectly innocent, but i knew nothing of his calls to her. if they were just friends, why didnt he tell me?
luckily she lived 2 hours away, so nothing actually came of it, but i am sure if she had had her way she would have tried to be with him.
i know he is not innocent in it, and do think that he knew what he was doing was wrong, but i put the emphasis on her, saying i trusted him, but not her.
for months after i checked his phone and phone bills, always questioning who he was talking to.
if i handt found out about it it might have led to something more.
but that was 5 years ago, we now have a 2.5 dd and i believe i can trust him.
i would say something to your dh along the lines of, 'i trust you, but not the other women out there, i know how women work and she must know this is not acceptable'iyswim
i hope you work this out, if there is something going on, you can work through it if you feel it is for the best in the long run.