I think you wait until the bill comes. I think you do nothing until then. You don't know what the bill will be. You don't know how it will go.
You know, this is a difficult situation. But, this is exactly the kind of thing happily married people deal with all the time. what would you do if you were happily married?
But maybe you could take a step back. What's the problem here? Are you really concerned about the money - is that what is important here?
Are you really inviting her to have her lover CALL YOUR HOUSE to save money??!?! Seriously? Is that the kind of person you are? "He can call here so that we can save money"???? Are you friggin serious? To save money you will encourage your wife to conduct her affair on your house phone?
What woman wants a man who makes that kinda trade?
To borrow a question I have been asking myself... How would you act if you were the man you want to be?
I know she is going to ask me to change the plan to accomodate her and I dont really want to do that. Are you sure? maybe you should wait and see.
If she asks, let her get her own plan. That seems fair. Have a talk about family budgets and what's a reasonable amount to spend on a mobile phone for a month. I'd say $65 or $70 is pretty fair, that's actually pretty high. Anything above that is over budget and is her responsibility.
You are you, and I am me. But if it were me, I would say: No, I am not subsidizing your affair. I am not paying $300 or $200 or $25 or 13 cents to subsidize your affair. What you do is your choice but I'm not interested in paying for your affair.