You per se won't be able to get her to do anything (you've already done quite a lot) It has to be something she does because she loves and respects you.
Cut off paying for the cell phone and her bills. Regardless of whether this is about sex or not, you still want her to be home and have a normal relationship. Try to find out what is it that these two individuals are providing her with that she feels that she lacks at home.
If possible, try to plan a weekend get away to a B&B, etc. away from home and take the time to really address this with her. She may feel stagnant in the marriage and looking for things to entertain herself. When she had married you, she had been quite young. She may be having a mid life crisis of sorts.