Engineer Job Offer with Fortune 500 Company After just a 30 min Phone Interview

Title says it all. I just had an informal 30 minute phone interview with the hiring manager of a Fortune 500 engineering company.

Hiring manager said next step would be a 3-person onsite interview.

Then suddenly today, I get a job offer without anything more than a 30 minute conversation. Should I be concerned or suspicious of anything?

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