The reason TalkTalk are saying you won't get your number back is because their CRM cannot specify a number to connect so just gives you a random number.
Depending on when they ceased your line and who the range holder of the number is, you should be able to get the number back somehow.
If it's a BT number range then talktalk keep hold of it for 10 days after which they then return it to BT.
If this has happened then the only way to get your number back is for talktalk to place a WLR provide order specifying the number you want and then migrate it back over to LLU
The problem with this is you're going to be without broadband until they migrate it back to LLU (unless you can convince them to provide an IPStream service in the interim) and if BT have already given the number to someone else then there's not much you can do other than convincing talktalk to ask BT to re-number the end user who currently has it.
If the number belongs to TalkTalk then again it will be quarantined for 10 days before being placed back into the pot to be used again.
The problem here is that your number could have been given to another customer so you would have to convince TalkTalk to contact said customer and convince them to re-number their line so you could then take the number. If the number isn't in use then talktalk would have to place the order manually, bypassing their CRM and request the number you want
Another problem is getting someone at talktalk to place your order back end manually as they won't be able to do it via their CRM and your standard TalkTalk customer service advisor will more than likely have no idea how to do this.
All a bit messy. You can tell who the range holder is by checking the number on the uk phone info website
Click on area code search and pop your number in, if it comes back with it being allocated to opal telecom then it's a TalkTalk number range