Bad luck! JustVoip doesn't support SIP. It is blocked essentially
. See the Voxalot provider list and also an old post on this forum. You have to use your credit for SMS now, or if you paid by Paypal you can ask for a refund through the Paypal website.
https://www.voipbuster.com/forum/viewtopic.php?t=11221
Edit: VoipCheap should work fine though. I have it working in Voxalot. Try a call VoipCheap to VoipCheap.