Hard time hearing the caller on speaker phone (Sangoma S500)

We had a Sangoma S500 telephone in a small conference room. We always had a hard time hearing the caller when we were on speaker phone. We purchased a yealink conference telephone. We still find that the call volume is low (even on max volume). The system is a FreePBX 60 with a 4 port FXO card (a200 I believe) with echo cancellation built in. I know that I can change the settings for the receive gain under dahdi config however I am not sure what I should change it to.

any help would be great

what happens if you call another extension inside the same pbx? does the speaker phone sound the same? what codec are you using?