ThinkPad T480 can be used as a fully featured mobile phone, supporting calls, SMS and mobile data. Here is a video of me receiving a phone call on mine (with the 25 year edition keyboard):

This laptop already has a SIM card slot for LTE modems. However, the PCM pins for audio transmission are not connected.

Luckily, thanks to an bug in Intel Management Engine, we can flash Libreboot instead of the stock firmware. This removes the whitelist for wireless cards and lets us choose our own modem. A good choice is Quectel EG25-G, since it was used in PinePhone and received some scrutiny. In particular, inside is an ARM core running Android (most Android phones have a separate Android system running in the modem). There is a custom firmware available with free software userspace. You set this firmware to register the modem as a USB audio interface and get the audio for phone calls. SMS and mobile data work out of the box.

M.2 versions of Quectel EG25-G (suitable for the T480) are quite rare and don’t have compatible antenna connectors. I needed conversion cables for the antennas, which were really finicky. But it can be done.

Therefore, you can now throw away your mobile phone and use a laptop only! You can even use Android apps natively with Waydroid.