My first thought is that you should definitely contact your ex to let him know the potential for lies coming his way. I'm not certain what your relationship is with him at the moment, but these are your kids that we're talking about here. Regardless, he doesn't have to be "involved" per se...but forewarned is forearmed and who knows? He might be able to offer you some suggestions or even support - exes are funny that way.

As for the phone, I'd contact the provider immediately and look at your options for getting the phone transferred into your own account. If you decide with your fiance that you're willing to work things out and things get better, it's still fine for you to have your own phone account.

In regards to your fiance...have you asked him about his changes? I know it's hard to deal with conflict - it takes me hours, if not days, of thinking about what I want to say before I have the courage to say it. But perhaps if you talk to him about the change in personality and behaviour, something positive might come of that. I think I read on your other thread that he is far away at the moment taking care of his parents - perhaps something is happening in that regard and he doesn't know how to handle things...hence the change. I'm in no way suggesting that his actions and attitude towards you are correct, but it might offer some insight as to the complete flip. Feel free to message if you need...