As a practical matter, is Tello enforcing this and cancelling lines that don't meet the requirement?
If you're referring to the purchase every 90 days, no one knows, since the first deadline for enforcement would be in December.
If you're asking about usage within each 90 day period, it's a bit of a mystery, but will become irrelevant if they enforce the purchase requirement. (A purchase counts as usage.) Some have had Tello send a text to their phones to avoid violating the 90-day usage rule-- but Tello has said they're not doing this anymore.
"- When will the 90 days begin? We start counting from your last *order* with us, or 10 Sept, whichever was last
- Will you start checking 90 days after the policy was enacted ie 8th Dec? We will not expire any numbers until 8 December"