I was also the same, i've moved my number from BT to TalkTalk, from TalkTalk to Sky and now its moving from Sky to BT with no issues.

In reply to a post by BuckleZ:

I was also the same, i've moved my number from BT to TalkTalk, from TalkTalk to Sky and now its moving from Sky to BT with no issues.

Was it LLU services you were switching between? Thats what tends to cause the problems. Just the fact that you lose BB for 2 weeks minimum is also an issue for me.
BT often waive their £130 "reconnection fee" when you go back to them, because its a way of getting you to sign their 18 month contract.

I was lucky that i was on SMPF so the changeover is fine. My line has always stayed on the BT side for the landline.