I agree it seems okay to me that you speak to your children on the phone ,I dont know of any studies but I do know that children do better when both parents divorced or not spend as much time as they can with hem, if your daughter is happy to be on the phone and it isnt disrupting any homework routine then ask your ex wife to be tolerant for the childs sake ...Good luck you sound very caring ...
I'm assuming you and your wife are divorced? This is a very sad situation. I see no harm in your children talking to you as long as they'd like on the phone, especially if they don't get to see you very often. If she's concerned that it's cutting into their bedtime or homework time then be flexible enough to work around that and tell her you will work with her on that if she will work with you on letting them talk to you as long as they'd like and that you'll make sure you don't cause them to go to bed late or cut into homework time. Good luck. I hope everything works out.