VOIP Call Volume

A quick question, if we recieve calls via PSTN volume level is fine, and I can always tweak tx/rx gain.

But when making a VOIP call the person other end complains that the call is quiet.

Is there an equiv of tx gain for voip calls? or any other options I can play with (short of changing the gain manually on all handsets, have 20 so rather not)

Setup is Linksys 941 talking SIP to Asterisk, which in turn talks IAX2 to ISP (has PSTN PCI card as well).

Thanks.