I am going to play devil's advocate here. First, you said he gave you the phone which makes me believe that he was telling you the truth that he was not contacting her. Now, given the benefit of the doubt here, what if she was sending text messages to him just to piss you off? Do you not think that this is all a game which she wants to destroy your marriage? Maybe he did tell her that he was done with her and accidentally told her he was going fishing or one of his friends told him he was going fishing. Either way, she knew and she knew exactly what to say to get under your skin.
Could it be that she is causing problems between the two of you? What did he say when you told him about the text messages? Was he angry with you because he told you that he didn't communicate with her or even seen her? It is so easy to jump to conclusions because you had been hurt. I know because I've been in that position before. It is so easy to jump on your spouse because of what you read. I know because lately I had been getting those same messages when my wife left me. It was hard for me to accept and I was angry and hurt. I know how it feels but sometimes you have to take yourself out of the picture to see what it looks like. I can see it if you were to jump on him because he hid the phone from you.
Now on the other hand. He could have given you the phone so he could create an argument with you. He may have seen her to end it at a bar if that is actually what had happened. The only problem with that is you do not know when he went to the bar, and who was present. Sometimes, you only get one side of the story and do not know what is actually going on. He may have told her he was done and she didn't get the message or he could actually be having an affair. The only problem is that you do not know and now that you got mad you will never know.
What you should have done was call her on his phone while he was laying in bed next to you. You could have seen his reaction when you tell him that you wanted to call him. If he was over her, he would have said, go ahead. Or you could have told him to tell her, "my wife left me, she is gone, do you want to come over?" Then when she shows up, you can come out of the bedroom and rip her a good one when she shows up then tell her to get off your property or you will have her arrested. Either way, you can see his reaction and decide for yourself if she is one of those who will not accept "its over". Then you can hear him tell her to her face, it is over and to never contact him again. Then have him change his phone number. If she contacts him again with his new number, then you can confront him again. But beware that if he gives out his number to any of his friends, they could in turn give that number to her. So you have to be careful to not think that he is cheating or giving out numbers. Sometimes you have to play the "what if" game to make sure you are not jumping to conclusions.
Sometimes emotions can take over and you can overreact. I guess for guys, we think out rationally before we react but still the reaction can be harsh since we have a tendency to want to seek and destroy. However, it takes a while to get to that point but once we do, its over.
Anyways, I hope you do not feel that you are alone. I've been cheated on so many times in the past. It seems like I never met anyone who didn't cheat on me. I wonder why. Guess it has to be me.
@Needstrength:
I agree with you about cheaters. It is hard when the person who cheats ends up with everything and you get nothing. My current wife filed for divorce and she was the one who cheated on me. Yet, she cannot even name why she is divorcing me. I wasn't the one who cheated, yet I am the one who has to pay? How is that fair?