I don't know where you live, so I don't have any recommendations for you -- BUT -- FILE A COMPLAINT!!! If you have insurance, file a complaint w/ your insurance company so they have information that will show them he needs to be removed as a provider under thier plan. Also file a complaint w/ the American Dental Association (or whoever it is) that he has his license through. A dr being distracted and answering his cell phone could have lead to serious issues! He could have hurt your son. I would absolutely file a complaint and not just let it go -- unfortunately, that happened to your son, but don't let it go and let it happen to someone else's also.
And, NEVER be a afraid to speak out during the procedure! If you saw him doing that, don't be afraid to tell him to not answer his phone, or to stop until your son's pain meds have kicked in. Don't be afraid to stand up to him, he's just a dentist - YOU'RE the MOM! You are your childrens advocate. I know it can be intimidating to say something to a "professional" - but you have to advocate for your child!
Good luck to you and I'll pray that your son overcomes his fear of the dentist. I'm so sorry that happened to him!
--S.