VoipBuster should work well. I noticed your call history (My account > call history) shows failed calls, this was because the FlyNumber wasn’t active yet.
Please try now and you should be OK.
Also, not sure if you’re using their app however using a 3rd party SIP/VoIP app (like Zoiper or Groundwire) with their SIP credentials may work better due to a limitation with their native mobile applications.