The logical conclusion of this, if it would work, would be that I can take my UK phone (Vonage VOIP) and plug it into my netbook when I am abroad and then I can get my normal call rates to UK landlines.
Seems to be an illogical conclusion to me.
Let me get this straight. The whole reason why you want mess around with your laptop is in order to enable you to plug your phone into it so that you can make phone calls with your phone over VoIP.
What on earth is wrong with just installing VoIP software on your laptop and buying yourself a decent headset ? Or plugging a hardware VoIP phone into the local switch/router ?
I simply don't see what extra benefits you are buying yourself by messing around with the mobile phone tethering element ? Something which others have already pointed out is only likely to yield questionable performance and call quality.