A few things:
-Take a look at this, and see if it helps
Setting up your SJ Phone softphone to use Voxalot - Voxalot FAQ
-Considering your situation, it makes sense to me to have:
Symmetric NAT handling (under Member Details on VoXalot) set to Yes + STUN server if possible on SJPhone
-Also take a look at this (just steps 1 & 2 there):
HowTo: 6 Steps To Optimize Your Audio - Voxalot FAQ
Are you really attached to SJPhone, if not I would try XLite and/or Zoiper (I've come to like Zoiper when out of the house, it is very straigtforward to setup, and I've had good results so far...)