So, when you changed the 'Phone', are we speaking of the handset, or the entire console? If only the handset, did you use a KNOWN GOOD Mitel SX50 console compatible handset? I ask because most handsets, especially from the majority of phones used in motels, are not fully compatible with the SX50 console. And where did the handset that was originally on the console come from. If they both came from other phones in the motel, you are not done troubleshooting. With all due respect to fifty5footer, I have never had a volume problem on an SX50 console caused by the backplane/motherboard (MCC). Not in 17 years and 200+ SX50 systems.
The SX50 console standard handset picks up surrounding noise and creates such a loud sidetone that it drowns out the incoming callers voice. Amplified handsets are of little use because they amplify everything going to the receiver, including the sidetone. I have had best success with Walker high output, confidencer handsets built specifically for the SX50. These have an extremely narrow pick-up cone for the transmitter and eliminate most of the extraneous noise that is typically at the 'front desk' of a motel. And of couse you could have a just plain bad handset. But, you won't know until you try a KNOWN GOOD SX50 COMPATIBLE HANDSET. IMHO John C. (Not Garand)

When I was young, I was Liberal. As I aged and wised up, I became Conservative. Now that I'm old, I have settled on Curmudgeon.