I have PbX QUADRO 2x and 16x (FW 5.3.26/28) and Telephone Mitel 6865i (FW 4.2.0.2023). The Telephone is registred as internal extension. Outgoing internal and external calls from this extension is without problems, but incoming call is not possible. Other caller extension gets a busy tone with SIP message 486. The same behavior is on both PbX. Other extensions with Grandstream and Gigasets work normally. Please, can there be any problem with setting the PbX or Telephone parameters?