Multiple SIP Trunks on UC540

Hi there, I got a UC540 the other day with a couple Cisco phones. I already had two SIP trunks with an ITSP local to me. The trunks both have their own number and authenticate with the ITSP using a username and password. I configured a trunk on the UC540 and after a bit of configuration it registers fine and I can make calls through it and I configured an inbound dial plan so I can receive calls. This is working fine but I am trying to conifigure the second SIP trunk. If I add a second username/password to the first trunk both of them register fine but if I try to create a DID dial plan rule using the second number I just get a fast busy signal when dialing in. The debugs on the UC don't show any activity so the call doesn't seem to be reaching it at all.
What I am wondering is, do I have to configure a separate SIP trunk for the second DID and if so, how do I do that. I have worked with Cisco stuff for a long time but have never worked with the VoIP stuff so I am not sure what needs to be done here. Is there any reason why haven't the second authentication through the primary SIP trunk won't accept calls on that DID.
Any help with this would be greatly appreciated.
Thanks