Exchange 2016 Receive Connector Configuration

2016 exchange server should receive phone voicemail from voicemail@clientdomain.com from Allworx phone system and deliver to users Outlook. Both on the LAN, phone server is 192.168.25.15 and exchange server is 192.168.25.10.

In the Allworx phone system it's set to TLS and username/password for credentials. I changed the voicemail@clientdomain.com password and updated in the phone server. Pretty straight forward however when I test from the phone system it fails and all the voicemail messages are waiting in the phone system queue.

I expect this may be the receive connector in exchange however when I configure a new connector it gives an error that IP address (192.168.25.15) is being used on the default connector and can't be used again.

I prefer not to change the default exchange connector (unless recommended) for the sake of causing email interruption outside the phone system. It's likely a receive connector needs to be configured for the phone server to work. Can anyone shed light on which configuration method to choose where I won't get the error IP address already in use by default exchange connector?