Yep, he is wrong to continually contact her regardless of the method of contact. But as others have said, even in a marriage there has got to be some expectation of privacy. You should not have been snooping in his phone.
Have you tried sitting down with him and talking to him about how his communications with this other woman make you feel? That would be the place to start. Maybe begin the conversation by apologizing for snooping around in his phone and then going from there, telling him how you feel about his contacting this other woman so much.