Disabling WWAN completely disables PinePhone modem (with ModemManager)
Plasma Mobile offers a quick toggle for mobile data, which ultimately (through 2 layers of indirection) calls the NetworkManager D-Bus daemon to set the wwan_enabled property. (I have that mobile data toggle disabled most of the time because mobile data is metered (0.009€/MB).)
Unfortunately, when it does this with the ModemManager backend, NetworkManager completely disables the modem, also breaking regular voice calls and SMS messages. With the ofono backend that was previously used by Plasma Mobile, disabling mobile data would leave voice and SMS working just fine, the desired behavior on the PinePhone.
Disabling the modem completely when disabling WWAN sounds reasonable on a desktop or notebook with a GSM stick, but not on a device whose primary purpose is to make phone calls.
This results in this Plasma Mobile regression: https://invent.kde.org/plasma/plasma-phone-components/-/issues/133
The workaround is sudo mmcli -m 0 -e to enable the modem (which thankfully leaves data disabled in NM, at least according to nmcli), but I have to do this every time, which is a major annoyance.