Ahh yes, but my issue with porting is the in the way it works - your number is never actually really moved to the destination network - it is just routed from its original network. So for example if you ported from Orange to Voda, to O2, any incoming calls are routed through Orange to Voda and finally to O2 - thats three different networks. I know there was an incident in the last few months for example were I think Orange numbers ported to Vodafone were having real issues for a few days (ie unobtainable tones etc.) - and of course nobody wants to admit the problem is theirs with each network blaming each other, and of course your old network aren't interested in talking to you anymore because you are no longer a customer.
In your case it is a bit on an exception since the number never leaves T-Mobile (since Virgin is of course a VMO on T-Mob anyway).
EDIT: Meant to say that the "port out to Virgin" was the workaround for porting out and in to Orange again too.