UC520 can support multiple SIP Trunks. Using Credential CLI you can configure multiple username/password for registration with the realm being the distinguishing factor.
On the IOS you can create several dial-peers specifying different SIP Registrar/Proxy servers under different VoIP dial-peers.
Under sip-ua mode you will need to enter the credentials commands for the different providers if you have to be authenticated. When you will be challenged by the SP, you will answer with a REGISTER message using the corresponding credentials depending on the SP domain name.
Configuring SIP AAA Features:
http://www.cisco.com/en/US/docs/ios/voice/sip/configuration/guide/sip_cg-aaa_ps6441_TSD_Products_Configuration_Guide_Chapter.html