I can barely hear callers when I use my DL72350 phone

I assuming you mean you can't hear well when using it linked to a cell phone. Do you have a phone line hooked up to it and how does it sound. Page 46 of the manual found here addresses volume when using the linked cell phone. Main thing it says is to set the volume of your cell phone. There may be other places that address the call volume when placing a linked cell call.

From manual:

If it is a cell call and the volume is too loud, too quiet or drops out, try changing 
the volume on your cell phone. On some cell phones, changing the volume on 
the cell phone affects your cell call volume on the DL72130/DL72210/DL72240/
DL72250/DL72310/DL72340/DL72350/DL72510/DL72570/DL72580 handsets

I tried that already and it didn't really help, it may have made a slight difference but I'm not sure.  I can still barely hear callers, there's nothing wrong with my hearing and it's the same in the other handsets that came with it.

Then it is what it is. I've never used the cell to phone link. Call the support number in the front of the manual, other than reading trough the manual I have no further ideas. If you have your cell phone maxed and the handset maxed I don't see what more you can do. Is the cell phone within 15 ft of the base unit? What make cell phone do you have?

I'll just return it, I was hoping there was some way to adjust the volume on the phone but obviously despite not being a cheap price it's a worthless piece of garbage.  They make cheap stuff like this and have forums like this in the hopes that people will hang on to it and keep looking for answers so long that it becomes too late to return it.  Clearly there is no fix for this, it just isn't any good but I kind of figured this which is why I didn't order it online and get stuck with return shipping.