Fax extension on VPS

Everyone! Thanks for help on fax receiving in a couple of days ago.

Now received fax well on PC and arm device, afterwards I installed debian 8+ freepbx 13 on Vultr VPS, everything works well, except receiving fax. I set a virtual extension with followme to “fax recipient” to virtual extension and enable fax receiving by UCP, but this extension always busy when dialing this extension number, anything wrong? your help would be appreciated.