To increase the call volume during calls, you need to add a couple of lines (on which you need to increase the volume) in the /etc/asterisk/extensions.conf file, at the beginning of each dialplan:
exten => X.,1,Set(VOLUME(TX)=5)
exten => X.,n,Set(VOLUME(RX)=5)
The minimum value is 0, and the maximum is 10.
For example:
[daytime]
exten => s,1,Set(VOLUME(TX)=2)
exten => s,n,Set(VOLUME(RX)=2)
exten => s,n,DIAL(SIP/204,30)
exten => s,n,Answer
exten => s,n,Hangup()
If you use Dahdi, you can increase the volume on the board by typing in /etc/asterisk/chan_dahdi.conf or in the context of the desired trunk:
rxgain=2.0
txgain=2.0
Here the standard value is 0.0, and the maximum is 6.0, where rx (receive) is the incoming volume, tx (transmit) is the one coming from Asterisk.