If you have lost the Internet Tethering (Personal Hotspot) option on your iPhone after putting in a TalkTalk SIM then this may help to resolve the issue:
1. Get the latest iTunes on your home computer (PC or Mac)
2. Connect your iPhone with your TalkTalk Mobile Sim and you should be prompted to update carrier settings from iTunes
To force this carrier update prompt, I had to first put in a Sim for a different provider. I used Three PAYG Sim and then connected the iPhone to iTunes. Disconnected from iTunes then swapped back to the TalkTalk Sim and then reconnected to iTunes.
What this carrier update did was cleanup the APN settings on my iPhone and loaded them from the TalkTalk Sim. However these were still incorrect for Personal Hotspot.
3. Then go over to the Settings-->Cellular-->Cellular Data Network
4. For Cellular Data, set APN to mobile.talktalk.co.uk and blank out Username & Password
5. Scroll down to Personal Hotspot and set APN to mobile.talktalk.co.uk and blank out Username & Password
This should then enable the Personal Hotspot Menu on an iPhone.
My understanding is that the iPhone checks if tethering is allowed by attempting to access apple.com via the Personal Hotspot APN specified in the carrier bundle. The Personal Hotspot APN on the sim TalkTalk provided me was not configured for TalkTalk but for Vodafone's APN as TalkTalk are a Mobile Virtual Network Operator on top of Vodafone.
If the iPhone can connect through the tethering APN, the "Personal Hotspot" function is enabled. If the iPhone can't connect, Personal Hotspot is disabled.
