I know it's frustrating. As hard as it is, don't call him or text him. This will be viewed as pressure. If he calls just listen, be pleasant and don't say too much. Don't try to stay on the phone with him. Be cordial but brief.
If your kids ask to call him, let them of course. If they don't ask, don't tell them to do so (also pressure). In the early days, my h had his phone off all the time and did not return calls to any of us. Sadly, my kids stopped trying to call him and one day S13 told him he stopped trying as the phone was always off. (Don't try to fix this relationship between the kids and him, let it run its natural course. It is not your problem to fix.) My h now reaches out to the kids occasionally. With s11 he has gone retro by installing a landline!!! Oh, MLC is so weird.
In those days, I turned off my cell phone completely once my kids were home. I grew tired of worrying and for waiting for texts/calls that never came in. Find some way to keep yourself from calling/texting.
This is the space and time he will take no matter what. This is where you let him twirl in the wind.
Me 41, H 47, M 15 yrs, S11, S13 BD 1: 11/4/14 we work on it; really I pretzel myself BD 2: 3/31/15 H goes down to "dorm room" 8/15: H back to MBR 10/15: H back in dorm room 1/18: H files, now divorced