Play Audio from a Mac over an iPhone Call
You should be able to do something like this with a product like IK Multimedia's iRig (or a generic equivalent... of which there are plenty). 



The iRig splits the shared audio input and output on the iPhone/iPad headphone port. Plug a patch cable from your Mac's headphone port to the input on the iRig (you might need adapters to go from mini headphone to standard... 1/8" to 1/4").