Even though Plasma Mobile on Pinephone is getting better and better, bugs can still happen. One of them is the loss of connection with the modem and thus the inability to make / receive a call and handle text messages.
How can I use the modem again? One solution is to restart the entire OS, but it is quite time consuming and tedious. A much easier way is to restart the Ofono service itself:
sudo systemctl restart ofonoctl
The solution was found on the Manjaro forum: https://forum.manjaro.org/t/restart-calls-plasma-mobile/53603
oFono is a free software project for mobile telephony (GSM/UMTS) applications. It is built on 3GPP standards and uses a high-level D-Bus API for use by telephony applications. oFono is free software released under the terms of the GNU General Public License v2