For the last 4 years, I've used a tiny M5 Phone in addition to my fancy Android device. This credit card sized dumb-phone is awesome! I use it on Shabbat when I want to be effectively off-line yet still connected for emergencies. I carry it in my man-bag as a backup phone. I use it traveling and in the back-country when I want to receive calls and texts from Shira, but want to save battery on my Android phone by leaving it in Airplane mode.
The M5 phone itself is cheap (less than $20!). The primary catch is my cell provider's monthly service charge. For the last few years, I was able to get away with adding an extra line to our standard T-Mobile account for $9.99. But recently, we had to upgrade our account and we lost this benefit. Sure, I could add an extra line, but it was going to cost at least $20/mo. Suddenly, my backup phone was looking like an extravagance I was going to need to forgo.
The challenge is my backup-phone's usage is so non-typical. I want tiny amount of talk and texts and no data; whereas cell providers focus on offering unlimited talk, text and data. While T-mobile had an antiquated text and call plan, it was still overkill and expensive.
I looked into prepaid plans, but found those to be even worse in terms of pricing.
But there was hope: through a Google search I found US Mobile. US Mobile let's you build a fully customized plan, including opting out of data and selecting just a few minutes and text messages:
A bare bones plan for $3.50/mo? Yes, please!
I ordered their SIM Card starter kit, inserted the SIM into the phone and I've had flawless service ever since. Of course taxes and fees mean that I'm not paying $3.50/mo - it's more like $7/mo. But still, I'm delighted that I was able to find a phone plan that fit my needs.
Another scenario where US Mobile could come in handy is for an Internet-of-Things experiment. I could imagine powering a project that works over text messages using a $2.00/mo US Mobile plan. That would provide an amazing capability for a remarkable price.
Bottom line: if you've got esoteric cell phone needs, US Mobile is your friend!