When a plan expires (not renewed or failed payment), the account and associated phone number can remain in an expired/suspended status for up to 90 days. In other words, the customer still has control of the account and phone number during that time, without active service. If not reactivated within 90 days, the account is deleted and the phone number is lost.
Currently, if you don’t care about losing your current plan, you can actually hold the phone number up to 180 days, by changing to the 90 day txt only plan.
The service would technically be available for the first 90 days, but the next 90 it could sit inactive if AutoPay is removed, and available balance is below the cost of the then current plan.
Why not let the plan expire for 90 days first, buy 30 day texting plan for $20, then let it expire for another 90 days? That would retain control of the number for up to 210 days, and it would be cheaper than 90-day texting, which is $55.
**Note to OP: Plan changes on an expired account must be done by a moderator, so don't actually wait until the 90th day to reactivate. Give yourself some margin against unforeseen snags.**