Now that ProjectFi has been rebranded GoogleFi and Google has officially opened up the list of devices that can be used on the network (subject to tmobile coverage only) they are running a promo today only for $200 in credits to bring your own device that was not purchased from Fi or google. Seems like you have to pay for the first 2 months before the credits arrive.
Phones such as the iphone 6s, oneplus 6, samsung 9 etc are all acceptable.
Full terms:
$200 Fi Service Credit with Compatible Phone Activation on Google Fi Promotion Terms
Limited time offer applies to any customer new to Fi who signs up for Google Fi between 11/28/18 8:00 AM PT through 11/28/18 11:59 PM PT, activates their compatible device that was not purchased at fi.google.com or store.google.com by 12/28/18 11:59 PM PT and remains active for 30 consecutive days. See eligible phones here.
To qualify for the service credit, activation must be for full service (i.e., activation does not apply to a data-only SIM). Afterwards, a service credit will be applied to the purchaser’s Fi account. Customer will receive an email notifying them of the service credit after all terms and conditions have been satisfied, and the credit will be applied to the customer’s subsequent bills around 45 - 60 days after activation. If the account is paused, credits will only appear once the account is reactivated. Service credits will be applied only toward monthly service costs such as data, calls, texts, and device protection; they cannot be applied device purchase or financing.
New Google Fi users must transfer (port-in) their current personal number over to Google Fi during sign up. The number being transferred must be currently active and have been active with the previous carrier and the user since 8/28/18 12:00 AM PT.
Limit one per person. This offer is only available for U.S. residents ages 18 and older, and requires Google Payments and Google Fi accounts. Unless otherwise stated, offer cannot be combined with other offers. Offer and service credits are not transferable, and are not valid for cash or cash equivalent. Void where prohibited.
Do note you can just order the sim today and decide on activation later (you have until 28 Dec to activate but only today to order the sim). $0 cost but it will get your spot on the promo in case you do want to take advantage.
Limited time offer applies to any new Fi customer who signs up for Google Fi between 11/29/18 12:00 AM PT through 12/16/18 11:59 PM PT and activates their compatible device that was not purchased at fi.google.com by 12/31/18 11:59 PM PT. See eligible phones here [google.com].
To qualify for the service credit, activation must be for full service (i.e., activation does not apply to a data-only SIM). Afterwards, a service credit will be applied to the customer's Fi account. Customer will receive an email notifying them of the service credit after all terms and conditions have been satisfied, and the credit will be applied to the customer's bills around 3 days after activation. If the account is paused, credits will only appear once the account is reactivated. The service credits can cover a single subscriber's unlimited talk and text, data usage, and taxes and fees for one month. The service credits cannot cover international calling, regular speed data after 15 GB per month, device financing, device protection, group plan member costs, or activity Google Fi deems excessive, abusive or fraudulent.
New Google Fi users must transfer (port-in) their current personal number over to Google Fi during sign up. The number being transferred must be currently active and have been active with the previous carrier and the user since 8/29/18 12:00 AM PT. Re-opened accounts do not qualify as new accounts.
Limit one per person. This offer is only available for U.S. residents ages 18 and older, and requires Google Payments and Google Fi accounts. Unless otherwise stated, this offer cannot be combined with other offers. Offer and service credits are not transferable, and are not valid for cash or cash equivalent. Void where prohibited.