My business has multiple accounts that all want and need to communicate with each other.

If there was a kind of reseller that automatically added all existing extensions from existing sub accounts, that'd allow them to communicate with each other easily. This would make billing and managing easier.

Example below.
Added on / extensions added to account's callflows in UI
2017 Account 1: extensions 1003, 1004, 1005..
2018 Account 2: extensions 2003, 2004, 2005.. (adds 1003-1005)
2019 Account 3: extensions 3003, 3004, 3005...(adds 1003-2005)