[Serusers] UAC module: multiple PSTN gateways

Is it possible to specify multiple authentication credentials within the UAC module? What I want to do is access multiple PSTN gateways that each require different credentials. The basic purpose of this is to allow least cost routing. Example Provider 1 offers cheap calling to +44 (UK) Provider 2 offers cheap calling to +1 (US) When user dials a 44 destination ser should relay to Provider 1 gateway, which will require authentication using Credential 1. When user dials a 1 destination ser should relay to Provider 2 gateway, which will require authentication using Credential 2. Any advice would be greatfully received. Regards Cameron