idle rainy day pondering - cheapest way to have a phone number + a little data for a backup iPhone?

[Buzz]

We use our VOIP provider's app on decommissioned (and previously unlocked) phones via wifi only, and that works pretty well. No SIM needed, and no costs incurred until swapped back into regular cellular service. Only extra cost issue, is that some carriers' SIM's age out if not used for a period of time, and if/when recommissioning a phone there may be a new SIM/activation fee. Not sure what other service/plan you're using, but some providers have 4th or 5th lines for no add'l cost. Several years ago (pre-smartphone days) we had two free lines from Sprint that were great for guests, and trips to Disneyland, etc. I've seen freebie family-line deals recently, from Sprint, T-Mo, and some MVNO's, which may be a viable option.
==