I've been reading that many are now having issues with getting their iPhone working as a hotspot in the latest iOS 7.1
I don't know how accurate the report is, but at least one site says this was very much intentional.
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In the update, Apple has apparently disabled users from setting up Personal Hotspots on carriers who either aren’t official iPhone partners, or who don’t support the feature. It seems that this is an attempt by Apple to crack down on unauthorized use of the Hotspot feature.
from: http://www.cultofmac.com/270871/personal-hotspots-causing-problems-ios-7-1-users/
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You on 7.1 and still able to use as hotspot?
On 2D?
On Telecom?
On Vodafone?
Cheers