I'm with Nanc1970, Talk to her and see if she'll tell you how she feels about what her father is doing. Taking the phone away is a good move, but she may resent you for it. When my daughter was 6 my ex and I divorced. a few months later my daughter was telling me that she hated me and wished I was dead, she was also abusive to her then under 1yr old sister. I took her to a therapist, who mostly talked to her while she played with toys and was told that my ex had been telling her that it was all my fault that we were divorced and he wanted to come home but I wouldn't let him. I also made excused for him when he was supposed to pick her up but didn't. (you're daddy loves you and wants to get you but he has to work), by making excuses for him I was transferring her anger onto me. so I stopped making excuses, handed her the phone when he said he couldn't pick her up and the counselor taught her to punch a pillow to release anger . The pillow was named daddy. Things got alot better. Maybe try finding a counselor for her to talk to, take the phone away (your divorce papers say nothing about him being able to talk to her whenever he wants), Talk to her, tell her that whatever she's feeling is okay and the therapist gave my a book called "How to talk so children will Listen and Listen so children will talk."I don't remember who wrote it but is was helpful for me. Good Luck
First of all my advice since I deal with a jerk for an ex as well....I completely understand...but you are in a little better situation than I am because you have sole custody. In my opinion I would get rid of the phone...turn it off hide it whatever you have to do to get it out of your daughter's life....she certainly does not need the negativity she is getting and as mothers we need to protect our children from all harm...physical, emotional, psychological..etc. This is the first thing I would try just try to ween her out of the control of her father's negative words....then see how her problem at school goes. If she continues you could have her seen by a doctor...she could have a bladder that is just smaller than usual that she needs to grow into...but my guess is it is something psychological bothering her. Be sure she is not being bullied at school, which could be causing it.
But like I mentioned my first thing would be to get rid of the phone and have her seen by her MD. Since she is in a stable home I don't think taking the phone away will be that big of an issue. She may even be glad you did, I'm sure as an 8 year old girl she is confused as to how to handle his abusive words....and she shouldn't have to handle it...that is what moms are for. We love our children and need to protect them at all costs.
Good luck and God Bless:)
Nanc1970