Hi,
Your right you can only have one sip-ua and I am not aware of any real way to hack that.
However you do have a few options. If you have an account with a VSP with say three numbers each with different username/passwords that you can have the one sip-ua and authenticate the username (and thus the phone number) at the dial-peer level. That way you can have a several numbers.
If you have two different VSP's then you will need a SIP proxy/registrar server like ser
http://www.iptel.org/ser/. With this you can authenticate the CME with the ser and the ser can have a number of SIP trunks ie redirect your calls. I am just in the process of setting up a test.
Hope that helps.
openeyes