Thanks for the update! That's good to know, especially for folks who aren't existing Tello customers.
Tello hasn't publicized it much, but existing Tello customers can get a free GSM SIM anytime they want to migrate over from the old Sprint network to the T-Mo network. (Just sign in to your account and go to the "Bring your phone" tab and fill out your information.) They have several shipping options depending upon how quickly you need to get the new SIM, but I got mine with free delivery via USPS in only three days.
One other "plus" with the GSM SIM cards is that if you ever swap phones you can use the same SIM... a nice change from all the different SIMs Sprint used to require.
Tello FAQ indicates GSM sim expires in 3 years:
(under CDMA sim card section)
What's the expiry date of a Tello GSM SIM?
The SIM is valid for 3 years since purchase date, so you can activate it anytime in this period. Still, we advise you to activate the SIM card as soon as possible in order to enjoy Tello's advantages & savings.
Suppose I sign up for Tello's current 50% off promo on their cheapest plan. I will get their GSM sim. Suppose I cancel the plan in one month but hold on to the sim. Will I be able to activate this same sim on Tello sometime in the future? Or will I have to get a new sim?
Edit - I am not a Tello customer currently. Don't have any lines with them.
It depends. In the past, I've rectivated previously active TMO proper and Republic Wireless T-Mobile network SIMs. Both needed to be reactivated within 20 days or otherwise needed to be replaced. A Ting TMO network SIM and Mint Mobile SIMs were one and done and once deactivated needed to be replaced. Like you, I'm not currently a Tello customer, so maybe its different.