Well, fixed installations of sound-powered telephones do typically come equipped with hand cranks to activate the "growler"/signaling system at the destination station.
However many sound-powered telephones are portable (as in, headphones and a microphone set with a long cord that you could walk around in). These portable sets would be quite unwieldy with a growler-style set attached, you'd have to crank throughout the conversation to achieve amplification, and you'd also make each sound-powered telephone set that much more complicated and expensive.
But the reality is that usually volume isn't that bad of a problem. If the circuit is quiet the solution is always the same: SPEAK LOUDER. Eventually you get used to being loud when needed.
