Ok, it sucks that your MIL is like this. It sucks mostly for your kids- but you know what- you cannot let this make you crazy. Just don't. If anything, your MIL is the one who is missing out on knowing her lovely grandchildren.
Sure, it is sad that your little ones don't understand why grandma doesn't call them back. But your sons are old enough to understand that even though grandma loves them, some people just have trouble expressing that.
It sounds like your children are being raised in a home with a lot of love. YOUR mother sounds like a very involved grandmother. I hate to say it, but unless YOU make a big deal about your MIL's lack of attention, my bet is that your kids will just not really notice or care that much, because they have an active grandma in their lives.
Like I said, it is too bad. But your MIL is the one really missing out. It sounds like when she is around, she is not really a positive member of the family anyway. Just let it go- if she wants to be involved she will let you know.
Otherwise, just focus on your wonderful family and enjoy them. There is no shortage of love and attention for your kids and you cannot change someone else's bad behavior, so don't stress out trying. It isn't 'right' but you don't need to be burdened with it. Good luck and happy holidays!