well, I am going to go against the flow.
I think that I could forgive and move on from one mistake. I do think that people make mistakes, do stupid things they regret later etc. And for me there is a difference over a one time thing and an ongoing affair.
So I wouldn't assume this was the end and that he was a cheating bastard, but rather I would sit down and take a good long hard look at him and us and decide if this was an isolated incident or not.
I think it is quite likely that the marriage has prompted the confession if it was a one off and he has been chewed up over it since.
BUT I would have to be sure that it was a one off and that it was out of character.
The phone stuff is very worrying. It was an emotional unfaithfulness so to me would count as another woman. That makes 2 times. Which is a pattern.
My only other question would be what was going on between you at the time, if both of these were around the same time. What was going on at home? That isn't an excuse, you don't want a man who buggers off and sleeps with someone as soon as you have a rough patch, but just to get a complete picture.
Whatever you do, take as much time as you need, don't be pressured into anything.