Put your phone on silent mode, and work until you don't feel like working anymore. Then check your phone whenever you feel like it.

Your phone calls don't need to control you. You can just check your phone later and see if you have missed calls / voicemails.

Working on something, but it's not super important and you don't mind being interrupted for a little bit? Take phone back off of silent mode. Easy.

Well, easy for me at least. Judging my all the comments here, this is absolutely IMPOSSIBLE for everyone else on Hacker News.