http://www.ukphoneinfo.com/locator

Each time an order is placed to re-point a telephone number, a "porting agreement" must be in place between the range holder 
and the gaining provider. Most providers have these agreements with most other providers, but there are still some gaps.

In your case, TalkTalk will have applied to the range holder of your telephone number for re-pointing of the number to their 
network. It appears that this order is failing, which is why your transfer orders have been cancelled and TalkTalk have 
advised they will need to provide you with a new number to offer you service on their network.