ARS picks trunks based on digits, not based on which phone is using it.
Each trunk has an extension under Devices and Feature Codes -> Trunks
To make a specific trunk an outgoing trunk for a phone, go to the phone, Associated Destinations, and set the Outgoing Extension and Emergency Extension to the trunk's extension. You can also put in the trunk group extension, if that group only contains the 1 trunk you care to use.