T25 with PBX Voicemail

I’m integrating a T25 into a PBX and want to use the PBX voicemail if the door station isn’t answered. I’ve setup an extension for the T25 to ring for 30s and then if no answer to go to voicemail on the PBX. However on testing the T25 just hangs up after the 30s. I suspect this is due to the very short transfer time between the PBX ending the ringing on the extension and transfer to voicemail.

Has anyone configured this successfully on a door station and been able to use voicemail on a PBX instead of the builtin messaging on the Mobotix?