I understand how fustrating it can be when this happens. My bf has done it a few times but i have also done it a few times. And everytime we done have done it is because we were just really really tired. I cant describe how it was for my bf but usually me, i rest my head because im so tired, i have no plans on sleeping through the night, just resting, but i ended up falling asleep.
If your bf is really tired, although it can be annoying because that's the only time you guys get to speak on the phone and spend time together, you just have to be understanding that he's really tired. Maybe call him a little earlier, or during the day if u can, or in the mornings. Just call whenever you can.
but i do get how hard it is, it makes you feel like they don't want to spend time with you or talk to you, for me, i just had to get over it and learn to understand.
if you guys both go to uni, maybe you can study on call together, that's what me and my bf do. or watch movies and youtube on discord together.
If you have expressed your concerns and he's still not putting effort into change, or find a solution then you might need to rethink ur relationship in terms of, if he ever makes scarfices for you, or just lives selfishly everyday. if he only spends time with u when its convenient for him then you guys need to talk about it