Looks like 12voip and voipbuster share their user database.

I've been able to connect and use voipbuster using my 12voip account.

I used:

User name: my user name@voipbuster.com
Password: my pass
Use for PSTN calls: I checked it off
Advance settings:
Connection
Transport: UDP
Outgoing proxy: sip.voipbuster.com
Port: 5060
Discover public address: I checked it off
Keep-alive mechanism: Auto
Keep-alive frequency: 0
Authentication
User name: Empty
Password: Empty
STUN
Auto-detect STUN: Unchecked
STUN server: stun.voipbuster.com
STUN port: 3478


Good luck!