Can you explain? I'm sure you don't mean the phones can't call each other, but I'm struggling to think of what you _could_ mean then. All the messaging apps I can think of are on both Android and iPhone?
Edit: more moderate phrasing
Caveat: I successfully did it with one group of friends. Have completely failed in all other attempts.
So uh, you guys _really_ like SMS, huh?
I don't care much for Whatsapp myself and prefer Signal, but I eventually gave up because life's too short for crusades.
Reading up on the iMessage adoption in the U.S.A. is like reading alternative-history fiction in Europe :P