Update (January 2108): TalkTalk appears to be winding down their mobile phone business. The provider recently began notifying customers with expiring or expired contracts that their deal would not be renewed.
TalkTalk has offered those customers to switch to O2 or another provider.
This looks like the end of the line for TalkTalk's mobile phone business.
If you are a TalkTalk customer, be sure to compare UK deals to find the deal that's right for you.