VOIPgain SIP Settings:

SIP port : 5060
Registrar : sip.voipgain.com
Proxy server : sip.voipgain.com
Outbound proxy server : leave empty
Account name : your VoipGain username
Password : your VoipGain password
Display name/number : your VoipGain username or voipnumber
Stunserver (option) : stun.voipgain.com