When does an iPhone become locked?

Hi

Do iPhones come locked to a network or does the lock happen when you activate the phone?

I bought an iPhone on eBay its new and still wrapped in the plastic.
The seller advised that it was locked to Orange and was an unwanted upgrade.
If I put a T-Mobile sim in the phone and then try and activate the handset, firstly will it work? and secondly will the handset at that point become locked to T-Mobile?

Alex

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