It is an incest manga. I can vaguely remember the mother talking on the phone to one of her girlfriends and her girlfriend suggests to the mother that she should give her son hand jobs to relieve tension as the girlfriend does it to her own son already. As the story progresses the mothers friend tells her to do more and more stuff to her son over the phone until the mothers friends son runs away because he asked her for sex but then the main mother gets a call from her friend saying that her son had come back and she is having sex with him and suggests to her that she should do the same.

Thanks.

First i thought that it was the Scarlet desire manga, but i think you search for Boshi no Toriko