An update from my side: we have decided to integrate modem and dsp partition images into the coming LineageOS updates. It may take a while for that to be approved in our case by LineageOS directors and I am still testing it here locally. This means that an OTA update will now contain these partition images in its payload:
boot.img dsp.img dtbo.img mdtp.img modem.img rpm.img system.img vbmeta.img vendor.img
The dsp, mdtp, modem and rpm images are new. I hope that this eliminates all remaining problems with reboots during calls for those users who are still using older stock images. I also expect this to work with locked bootloaders. Note that we are being selective about what to ship, and not updating everything (bootloaders, Keymaster, TrustZone) like a stock Fairphone update would do. If any problems pop up, please let me know. Keep in mind that the FP3 is an A/B device and can easily be rolled back to the working slot if an update goes bad for you.