How can I configure a Linksys PAP2T and/or dial a prefix on a phone
connected to Line 1 such that an outgoing call goes out on Line 2?

I've just set up voiptalk.org successfully for incoming and outgoing
calls (at ****ing last!), after being ditched by voxbone as they don't
support personal customers. However, I'd like to use voipcheap.co.uk for
outgoing calls.

I've configured line 2 for this. How can I use it from a phone connected
on Line 1?

Thanks,

Ian