I have thought of doing this for a while...but you don't want a VOIP application - you need the following:-
Digibox modem plugged into a Telephone Line Simulator (this is basically a little box that allows two modems to be connected to each other).
A PC with an analogue modem (e.g. 56K) connected to the other side of the Telephone Line Simulator. The PC needs to have Internet access, ideally permanent (like DSL with autoconnect). The PC needs to run an application that will take incoming serial traffic (from the modem) and send it via a telnet connection (probably over an SSH connection) to a PC in the UK.
The PC in the UK needs to be permanently conencted to the Internet and use a dynamic DNS service (like
www.noip.com) to allow the remote PC to find it. This PC then needs to run a server application to service the incoming connection from the remote PC (probably telnet over SSH) - the output from the telnet server should be directed straight to a modem on a local UK telephone line (for SKY Multiroom, the same line as any other Digiboxes).
In theory this kit would have to be up 24x7 but SKY only actually get the digiboxes to dial out once a month, apparently some fixed number of days from your SKY billing date.
Personally, I thought the above was too complicated and I would rather just pay up.
P.S. I forgot, there is also the issue of passing the number dialled by the Digibox to the remote system. This could prove to be quite difficult and one might have to settle for dialling a fixed number at the Uk end and hope SKY don't change it.