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Where did you purchase the phone? Was this an unlocked phone? Was this a new phone?
If the phone was carrier locked to another carrier (which is where this error generally comes from) you would need to contact that carrier to see if they offer unlocking, and if they do, do you qualify for an unlock. You need to provide a little more information for any further thoughts.
More than likely it was. If yours was with Vodafone and it is not accepting the Tescos SIM, then it is locked. You can contact Vodafone since you were a customer and ask them if you qualify for an unlock. If you do, follow their instructions for an unlock. Once that has been completed, you will be able to use it with the Tescos SIM.