Note: Not sure how the 2G sunset will effect this emerging product, but hopefully they have it all figured out. ~JDL
The Light Phone is a discreet credit card sized mobile phone designed to be used as little as possible. It is the only phone designed to be used as your second phone as a seamless extension of you smartphone. It is an unlocked 2G GSM phone that works independent of your current plan, but using our unique software platform it can keep your same phone number. The Light Phone is easily set up and controlled via an app on your computer. Other features are limited to nine speed dials and a time display.
An active SIM is required because the phone is not tethered, it will work regardless of where your smartphone is. The cost of using our service is approximately $5/month in the United States.
Read more here.