GoodKarma,
That's a good question. I had wondered that too, before it happened to me.
Mine would do drugs to get the courage to cheat. Then, sometimes he could perform, sometimes he couldn't. There was one woman who accused him of getting her pregnant. He swore up and down that it couldn't have happened, because he couldn't even perform. You can't imagine how much better that made me feel (sarcasm there)
Ask any of the women here who have suffered through unfaithful addicts, and you will see how they manage to cheat. It happens at the fringes, when they're just going up or just coming down. It happens when they're at full-tilt, when they can't even maintain an erection. It happens when they sell drugs to attractive young women & teens who don't quite have enough money. It happens whenever it happens, and it happens because they're out of control and many drugs make them want sex. Many of our addict spouses here have had problems with pornography and sexual deviance...even though they can't really perform. Doesn't seem logical, does it?
And as for the phone calls? My suspicion is that either he has unreliable sources, and has to call around to find what he needs, or that he's dealing. Unfortunately, probably the latter based on the number of calls you talk about.
Which means that, if you're caught with him and he gets stung, no matter whether you knew about it or not, you get taken to jail too. Just happened to an ex-employee of mine.
Please please please be careful and be tender with yourself. And please come back! There's a great deal of support and love and friendship out here, an amazing amount really (it always amazes me)
Take care,
GL