Putting myself in your ex's shoes, the reasons I would be ringing you are:
1. I am regretting my decision as I have realised that when the chips are down (i.e. miscarriage) my new man isn't as supportive etc as you were.
2. I want you to realise how unhappy I am so that you feel sorry for me and don't take my kids away from me.
3. I am lonely in my new home (assuming she is the one that moved) and I haven't made many friends and you are easy to talk to.
4. I don't want you but I don't want anyone else having you either.
Regardless of which it is, they are all selfish reasons, so unless you think it might be number 1 and you wish you were still with her too, I would try not to answer the phone to her very often, especially if she rings when you are you with your girlfriend. If the phone rings, let her leave you a message or cut her off then follow with a text saying 'I can't answer my phone right now, please text back if it is urgent and about the kids, otherwise I will call you tomorrow'. Alternatively, if your girlfriend is up for this, let your girlfriend aanswer and say 'sorry he is in the bath'. Above all else, put your girlfriend before her. My partner had an extremely bitter ex, esp given she was the one who had affairs etc and my partner constantly bowed to her for a quiet life. You need to nip this in the bud otherwise it could go on forever. We have only just got rid if the ex and it has been 8 years of hell! By the way, I may have put myself in your ex's shoes, but that doesn't mean I am the sort of person that would do any of the above numbers!!