There is a golden rule to gains - if the service works as you want it to, leave it alone otherwise you could screw it up. The only time you should need to adjust gain on anything is if a particular piece of equipment didn't 'hear' the dialtone for instance, such as a fax, DECT phone or PBX, and you needed to adjust the gain to 'turn it up'.
The only two times I have ever known gain to play a part is firstly about 10 years ago when v.92 modems first came out, and I was only achieving about 3.8k/s (and okay 5.6k/s was only ever theoretical, no-one ever reached it, but 3.8 was low). I asked BT to increase the gain on the line, which got me up to 4.2k/s, that was fine for me.
When I used to be on Nynex cable (now we're going back!), they also got the gain wrong on my 2.88k/s modem. That meant voice calls went through with no problem, but the modem failed during three handshakes and ended up connecting at 0.48k/s.
In other words you're not using a modem, you couldn't reliably even if you wanted to on VOIP, so it doesn't matter. Start to turn the gains up with no reason to and you run the risk of blowing components in the phone equipment itself.
