You may have only one port enabled on the Quintum in one end.Or the PBX could be only alowing you to make one call at the time.
I dependes on your configuration: If you have the PBX attached to the Quintum via the FXO interfaces, you will dial a extension number and reach the Quintum. Test at the originating site to see if your call is reaching the Quintum by pluging a single line phone on the line going into the Quintum and making a second call.That will tell you if the trouble is PBX or the rest of the system.
If your Quintum is attached to the FXS port, you will pick up a phone, push a specific line on your phone (or dial a access code) and get Quintum dial tone.
Verify ,using the single line phone that all four ports on the Quintum will give you dial tone.
If all this was already done, you could turn on the log into the Quintum, try to make the second call and see where it got rejected.
I could give you a hand if you need it, and I will not try to sell you anything.
You could reach me at nospamwboyrie@yahoo.com. Remove the nospam, start with the “w”
Good luck!!!!!!