I suspect the person wants to be able to resolve a domain name to an internal IP when looking up the address from the same network.
For that to work you probably need to setup a local DNS server for DNS lookups from the local network and setup the appropriate Zone files to return the proper results to local workstations.
You probably don’t want to be running bind on the same machine as your phone system. You should probably set this up on a different device inside your network and administer it separately.
In any case, the answer on how to do this would be out of scope for this forum. You might want to look at how to do that somewhere else online.
If you are wanting to change what IP address your PBX resolves an A record to you would add a line inside the /etc/hosts file on your phone system. This change would only affect your phone system though.