In order to use Jumblo service you need to properly configure it if you use another application or simply check the following settings to be accurate:
SIP port : 5060
Registrar : sip.jumblo.com
Proxy server : sip.jumblo.com
Outbound proxy server : leave empty
Account name : your voip username
Password : your voip password
Display name/number : your voip username or voipnumber
Stunserver (option) : stun.jumblo.com
Also, if you cannot place calls, I suggest you re-install the VoIP software. An activate Internet connection is required for a successful connection.