Can you try to help them understand what is behind their dad not calling. If the problem is alcohol or drugs, somehow it’s almost easier because there is something to blame.
If there’s no excuse, explaining that their dad is thoughtless and self-obsessed isn’t going to help. All you can do is keep reassuring them.
Knowing that the three of you are rock solid as a family unit is essential, And make the school aware of the issue, and ask for their support.
Can you set some things to look forward to, a treat in the summer or a city break somewhere new at autumn half term?
Being teenage is quite hard enough. I hope it gets better soon.