REALLY simple:

obviously the iPhone has good enough speaker volume if you can play music on it without headphones, all that is needed is a program that will route call volume through that same speaker system, rather than a very quite system with a small volume ceiling

I know this is essentially speaker phone, but why not start with that level of volume, and then allow the volume adjust buttons on the side of the phone to drop it to the acceptable speaker level.

This would be the default call setting