Hi B.,
You don't say how old the kids are, and that will make a difference. But it sounds like they are on the younger side? I agree that set times are the way to go. My ex didn't want to have set times - he just wanted to call whenever he wanted and she should just drop everything (dinner, movie, homework whatever) and talk to him and it made it REALLY difficult to do anything and then he would scream at me that I was withholding his daughter from him. So, I get it from that angle, but I know plenty of mom's who play games the other way around and there are dads out there that really do want to be involved.
Phone calls are difficult because throughout the week there is only a couple of hours between pick up after work and bed. And that's almost completely taken up by homework, dinner, bath and bedtime routine. A 15 minute call isn't a 15 minute call if it interrupts dinner or homework or there are emotions involved and she has to ramp down and then focus back to what she was doing.
Now - I ABSOLUTELY think a dad deserves to talk to his kids. My opinion is whenever... I think 2x a week is not enough if they are going to be an involved parent from out of state. But, it always seemed like the call came right in the middle of dinner, or right when she had started homework.
I also know that he would want to call sometimes and say goodnight and that is the WORST time because she was tired and it would get her all worked up. So, he would call every couple of weeks for a 30 second call at 9:00 and then she would be up in tears until 10:30. So, I had to say no to that, except on weekends when her bedtime wasn't as firm.
I finally, with the help of our mediator, got him to 'agree' to a 7:15pm call. I picked her up at 6 so we were done with dinner. She talked to him at 7:15 on Mondays and Thursdays and we made sure to work ahead on homework the other days so that she didn't have any and could spend as much time on the phone as they wanted. Usually they were off by 7:30 or so and she could do reading or homework and then start the bedtime routine around 8 with bath etc. That worked for us.
What made a WORLD of difference was when I got her a cell phone and I could just remove myself from the picture - except that I made sure she checked messages and missed calls and whatnot.
They also text a TON now - I would say it's their primary form of communication and it's probably daily, whereas they will only talk on the phone occasionally.
I would definitely say he needs to keep records of how often he is calling. And definitely for him to call back often. If he calls and leaves a message and they don't call back he needs to call back several hours later and again and again until he sets the expectation that he will get a return call.