By default no modem is allowed.
Property "bluez5.hfphsp-backend-native-modem" can be 'none', 'any' or the
modem device string has found in 'Device' property of
org.freedesktop.ModemManager1.Modem interface, e.g. for PinePhone
"/sys/devices/platform/soc/1c1b000.usb/usb2/2-1".