How to increase the sound volume on outgoing calls?

Hello,

Is there any way to increase the sound volume on outgoing calls. Users complain that when they place or receive a call, they can hear the caller very well but the caller can barely hear them. This only happens on SIP/IAX trunks to ITSPs, but internal SIP and IAX calls between internal and branch office extensions are fine. The codec used is ULAW. Are there any settings in the Asterisk configuration files to increase the voice intensity on calls placed on ITSP trunks?