JustVoip, VoipBuster, and FreeCall, all from the same company, use SIP protocol.
You should be able to connect with any SIP software.
ZOIPER IAX Softphone | SIP Softphone
QuteCom - Welcome to the QuteCom developer's home
Both should work (i have only tested zoiper).
I don't know of better SIP clients for mac.
Here is JustVoip SIP information:
SIP port : 5060
Registrar : sip.justvoip.com
Proxy server : sip.justvoip.com
Outbound proxy server : leave empty
Account name : your JustVoip username
Password : your JustVoip password
Display name/number : your JustVoip username or voipnumber
Stunserver (option) : stun.justvoip.com
If you have audio problems:
Use a STUN server (e.g. stun.justvoip.com) with port 3478 (if supported by your device)
Use the G.711 codec
Happy new year.