None, but the point is that trying to make a dumbphone, using Linux as an ingredient, allows each part of the product to be very specific about the other parts, whereas trying to add general phone support to Linux (say, if you plug in a USB modem) will require either a lowest common denominator or an inconsistent experience, and will have abstraction layers in the middle making the software much harder to develop. Consider that the SMS app for My Linux Dumbphone can open /dev/ttyUSB5 and send AT commands, and the experience as a whole is tested and edited by one person until it works well, while the one for KDE has to use some KDE API which either adapts to KDE Connect (which has to follow an already-designed protocol) or to a Linux kernel API which is implemented by a lowest-effort driver.