I'm wondering if there is a Dad around. (?)  Our children are so dependent on us for stability and structure, that unconditional love of a parent. I certainly don't want to put some sort of guilt trip on you, today we all have to work to survive, but I would bet that your Son just wants his parents.  He is still very young and probably feeling insecure for himself and his little sister.  He may also be bored, or feeling neglected by whoever is caring for him... then again he may be having a lot of fun and just wants Mom to be having fun with him!

I always worry about the future impact of things on children. I would say try to talk to him one on one as often as you can, but not on the phone while you're at work... it's obviously only upsetting things worse for both of you. (Tell him you can't take calls at work anymore). Someone will call you incase of an emergency right?  Then just try every chance you get to spend quality time with him and his sister... lot's of talking and affection.. try to make up for lost time.  It sounds to me like your child is just lonily and alittle lost right now, and doesn't know how to be grown-up about it yet.   Good luck to you.