It's not a word. Not in any language *I* speak. But the press has picked it up ans splattered it all across the HP leak scandal as if it's accepted English and possibly a technical legal jargon term. Which it is not.
When someone calls you on the phone and pretends to be someone else, yes, they are doing this under the pretext of being another person. But "pretext" is a noun for which there is no corresponding verb in English. The closest verb I can think of is "misrepresenting", though I prefer the more general "lying".
It is definitely illegal - it's called fraud.
So to the mendacious former head of HP who quipped to Congress yesterday that it was 'surprising that it isn't illegal' needs a serious reality check. It is illegal to obtain private information under false pretenses. And the follow-up is that this information was used to obtain private electronic records, which is definitely against the law - the 1986 Electronic Communications Privacy Act, to be specific. The penalty is a fine of up to $10,000.