FYI:
A Sprint tech told me, yesterday, that the one year free promotion would no longer be offered after January 31, 2018.
Hopefully, by then, there's more similar deals or major price drops. With LTE becoming the standard and 5G coming online, there will be plenty of free capacity that the carriers will want to "give away" to encourage subscribership numbers for their corporate reports.
https://www.sprint.com/en/shop/offers/free-unlimited.html?ECID=vanity:1yearfree
Fineprints:
** Mo. charges excl. taxes & Sprint Surcharges [incl. USF charge of up to 19.5% (varies quarterly), up to $2.50 Admin. & 40¢ Reg. /line/mo.) & fees by area (approx. 5–20%)]. Surcharges are not taxes. See Understanding the surcharges, taxes, fees and other charges on your bill.
Limited Time Offer. Credit approval req. Req. AutoPay, eBill, and port-in from postpaid carrier. 1 year free Unlimited Offer: Eligible device must be unlocked and compatible with Sprint SIM card and network. Reqs. SIM card purchase ($2.99 + $10 S&H + tax). SIM must be activated prior to port-in. No device upgrade allowed during first 4 months. Phone features and network experience may vary based on original device carrier. Excl. ports made between Sprint or related entities. Unlimited — Bring Your Phone Plan: Incl. unlimited domestic calling, texting & data & 10GB of high-speed Mobile Hotspot, VPN & P2P. Third-party content/downloads are add’l. charge. Sel. non-premium int’l. svc. incl. See Sprint Global Roaming. Up to 5 lines. Plan not avail. for tablets/MBB devices. After 1/31/2019, plan will become Unlimited Freedom. Add a Line: Add a line at any time with port-in from postpaid carrier and Bring Your Own Eligible Phone up to the max. number of lines & get promotional pricing until 1/31/2019. AutoPay: $5/mo. discount req. continued AutoPay enrollment. Quality of Svc. (QoS): Customers who use more than 23GB of data during a billing cycle will be deprioritized during times & places where the Sprint network is constrained. See Open Internet Information for details. Usage Limitations: To improve data experience for the majority of users, throughput may be limited, varied or reduced on the network. Sprint may terminate svc. if off-network roaming usage in a mo. exceeds: (1) 800 min. or a majority of min.; or (2) 100MB or a majority of KB. Prohibited network use rules apply—see Sprint Terms and Conditions. Other Terms: Offer/coverage not avail. everywhere or for all phones/networks. May not be combinable with other offers, including but not limited to AAA, AA, Credit Union, Premium International Experience, Port in credits, Visa Prepaid Reward Cards, Referral Rewards programs and IB/Lease promotions. Restrictions apply. See sprint.com for details. © 2017 Sprint.
Seems we are able to sign up by porting in from Freedompop. So is Freedompop prepaid or postpaid?
A question for those of you who have actually completed a full year on this free plan:
What happens on the 13th month?
Does every thing simply end or do you have to take specific action so that your credit card won't be billed "ongoing" at the regular rate beginning on the 13th month?
Doing nothing would be unwise. You will be converted, and billed for, something like $60 for the first line and $40 for each additional line on the same account. It's in the terms of service that you accept this.
You must either actively cancel the lines (and lose the associated numbers), or port out each number (which will automatically close each line ported out.) The paranoid folks definitely suggest porting out-- and I'm one of the paranoid folks when facing the possibility of $60/line.
To be clear, there is no long-term contract. You can cancel in month one, in the last month, or anywhere in between with no penalty.
It looks like the Sprint free year deal has been extended.
Their site now says "Savings until 4/30/2020"
They normally extend it 2 or 3 months.
Interesting that they only extended it 30 days, this time.
How are you figuring the extension is for 30 days?
Signing up a few days ago, the plan would have run until 3/31/20. Signing up today, the plan runs until 4/30/20.
Although the plan is referred to as "1 year free", it can vary from a minimum12 months to 15 months, depending on where you are in the current signup period. If they don't refresh the end date, a person signing up on 1/31/19, and 3/15/19, might have the same 'end date' on their plans.
Anyone taking the chance hoping the offer gets extended again? Or are you going to make sure you're all signed up to have year free to Apr 2020?
I locked it in this month. I'm good until 3/31/20.
I guess the important thing to remember is to make sure you also add add'l lines... I am just not sure if they'll let you add more free lines after April, despite having the account open in time. It should, but just not really sure.
Any more rumors on the end date to sign up?
Well, this thread was started 14 months ago, warning of the likely imminent end of the plan. It seemed believable. I imagine there have been more recent rumors, but only time will tell if they're more accurate.
Approval and finalization of the proposed T-Mobile merger will likely signal the quick end of this free promotion, IMHO.
https://www.prepaidphonenews.com/2019/03/proposed-t-mobile-sprint-merger-could.html
So does this mean the one year free gets extended another year? 
T-Mobile will likely allow those with this promo to ride it out and then give them the choice to leave or go on a conventional plan.
This expired deal thread is for free year ENDING 7/31/19.
According to @KentE post, most recent Sprint free year promo listed end date of 4/30/20. https://www.nthcircle.com/forum/big-four/1231-sprint-free-year-promo-end-date?start=110#35186
There is anecdotal evidence accounts already on free year promo can add additional free lines with same end date as existing line(s).