Phone's Ringer Decreases After Initial Phone Call

[monicakm]

Well DH's S5 has been doing this for months but he could never tell me enough to know where to even start looking. He finally told me tonight that when he turns his ringer up and gets a phone call, the ringer is where he put it. But, each call after that, the volume is all the way down (but not silenced) and he never hears it. I thought he as turning it down accidentally by the way he was holding the phone. I experimented tonight by turning the volume all the way up, called his phone and the ringer was full volume. Hung up and called him again. Volume was all the way down. Did this 3 times with the same results. Anyone know how to fix this?
Thanks