Late last year, Google Fi ran a promotion offering new subscribers a free month of service if they ported in a number from another carrier and brought their own phone. The company is back at it, as the offer is available again for a limited time.
The offer is available only to new subscribers — if you've used Fi in the past (even when it was still a "project"), that's not you. The number you're porting in must have been active since at least December 8 of last year, and you'll need to bring a compatible device that wasn't purchased through Google Fi. You'll get an email letting you know you have a bill credit within a week of activating your account.
You can get this deal right now. You'll have to get set up before 11:59 p.m. Pacific time on March 24 if you want to take advantage, so I guess don't sleep on this one for more than a couple weeks.