[Users] OpenSer as a UA?

Hello I'm struggling to understand how all the bits of a VOIP system fit together and whether OpenSer can act as a UA and authenticate to a remote proxy. What I want to achieve is this. Incoming SIP calls are first routed to a SIP phone on the network. If this is unavailable, the call is then redirected to another proxy of a voip service provider with whom I have an accout, whence it should be routed to my mobile phone. I guess I can rewrite the call URI to mymobilenumber(a)proxy.voipprovider.tld, but I will need to authenticate to the proxy to authorise the call and have it charged to me. Can I do this with SER, or do I need something else (Asterix?) for this sort of function? -- Chris Hastie