Faking a famous person would seem to me to be easier (for various reasons) than faking my colleague. It's not enough to fake the sound of their voice, it's also the manner in which they speak - word choice, attitude, responses, knowledge, sense of humour etc. But I'm guessing the target of this attack only knew the fake person they were speaking to marginally.

The approach seems to be unnecessarily risky vs just phoning up pretending to be someone they didn't know is my point.