had an old post on this from last year.
I just upgraded a client to a new FreePBX16 cloud based PBX (vultr) because their PBX hardware died. They have older Digium D40 and D50 phones. They were coming from FreePBX V14.
The phones are connected through EPM and DPMA. They ALL now display the behavior discussed in the old post. On initial dial out from any phone, you get a fast busy then must hang up and dial again. Second attempt always works. This happens dialing internally or externally.
I only see this on older D series phones (D40, 50, 70 etc) or on newer D series phones (D62, D65) when there are older ones present on the PBX.
This has got to be something with these phones and newer versions of FreePBX or Asterisk.