I'm making a replacement motherboard for the Nokia 1100 that runs Linux and supports LTE. I have the software working well on a MangoPi dev board and I've designed two PCBs: one "test fit" board that just breaks out the phone keypad/LCD to pin headers, and the actual board with everything on it. Now I just need to order the PCBA which is super expensive :)

It uses an allwinner T113-S3 SoC and the Quectel EC25 LTE module, and I've made sure voice calling works over LTE, at least on T-Mobile in the US. (this is tricky bc of carrier rules)

video and photos of MangoPi test setup and my board layout here: https://constcast.org/n1100c/