4135 Conference phone on OXO calls disconnect

[Xero]

Hi,

I have installed 4135 conference phone on OXO, everything is working perfectly ok with it outgoing calls, conferences everything but only one issue is that when I a caller dials the Master number which is mapped as a hunting number call lands to the autoattendant, and when someone tries to dial the extension number of the said 4135 IP phone it rings a half ring and disconnect. On any other extension the call is landing perfectly fine.

On this 4135 IP extension I tried to route a trunk directly and it is also landing on it. but only issue we are facing with when anyone dials the extension through AutoAttendant it disconnects.