Meet Zanco tiny t1, world's smallest mobile phone
Although it's just a feature phone, Zanco tiny t1 offers all the basic features you would expect from such a device, including calls and SMS. It's only working on 2G networks (GSM850/900, DCS1800, and PCS1900), but the very small 200 mAh battery lasts for more than 3 days on a single charge.
The tiny t1 sports a tiny 0.49-inch OLED display with 64 x 32 pixels resolution. It comes with a numerical keyboard, but your fingers must be really small for 100% accuracy. Also, the feature phone is compatible with nano-SIM cards and can store up to 300 contacts in the phonebook, as well as 50 most recent calls and 50 text messages.
If you want one, you can pledge a minimum of $50 for the Zanco tiny t1 on Kickstarter. The company plans to deliver all orders in May 2018, but that's just an estimation.