I am so sorry for your situation.  What your ex is going to say is that your daughter was using the phone at inappropriate times and that is was interfering with her studies, etc.  But you can not be denied access to your daughter and she has a fundamental right to have access to her siblings.  So you need to challenge your ex wife on this issue as soon as you can.  If you can establish that your ex wife is interfering with your ability to have access to your daughter and that you have purchased a cell phone to allow your daughter access to you, her father, at any time and that is with out interference to the parental rights of your ex wife,  then I think that the coutrs will be on your side.  Good luck to you.