As an aside, I hope you are not referring to Voicemail which is a different thing entirely/
Avaya 9620 is an IP only
Phone. This means it CAN ONLY be connected to an IP Phone system.
Excerpts from guide
'Which systems can you connect your telephone to?
This telephone is intended solely for connecting to Avaya communication servers. '
'These telephones are intended for connection to Ethernet/LAN interfaces of IP-capable Avaya telecommunications systems
and fulfill the basic requirements of Directive 1999/5/EU (R&TTE)'
'The design of these telephones makes them unsuitable for direct operation on interfaces of public telecommunications networks'
All this means is that you cannot connect your phone direct to a normal Telephone line.
Please read the beginnning of the guide here.
So , it looks to me that there used to be ONE telephone system for your two Companies and now you have split up, NO ONE appears to be taking responsibility for programming the AVAYA Phone system to which your telephone is connected. It MUST be somewhere
in the building !!!
Try and find it. IT will be here that the answerphone facility is programmed....definitely NOT