Start-up stalls getting own number

[Colin Helliwell]

Just trying out this morning's Head revision, and it seems to be stalling at the point it's trying to read its own number.
mmcli -L reports no modems, and the logs ends with (i.e. there's no further messages at all):

Jul 19 09:19:19 wg daemon.debug ModemManager[830]: <debug> [1500452359.141518] [src/mm-broadband-modem.c:1199] modem_load_own_numbers(): loading own numbers...
Jul 19 09:19:19 wg daemon.debug ModemManager[830]: <debug> [1500452359.141858] [src/mm-port-serial.c:1252] mm_port_serial_open(): (ttyMux1) device open count is 3 (open)
Jul 19 09:19:19 wg daemon.debug ModemManager[830]: <debug> [1500452359.142265] [src/mm-port-serial.c:1309] _close_internal(): (ttyMux1) device open count is 2 (close)
Jul 19 09:19:19 wg daemon.debug ModemManager[830]: <debug> [1500452359.142631] [src/mm-port-serial-at.c:463] debug_log(): (ttyMux1): --> 'AT+CNUM<CR>'
Jul 19 09:19:19 wg daemon.debug ModemManager[830]: <debug> [1500452359.176295] [src/mm-port-serial-at.c:463] debug_log(): (ttyMux1): <-- '<CR><LF>OK<CR><LF>'
Jul 19 09:19:19 wg daemon.debug ModemManager[830]: <debug> [1500452359.176799] [src/mm-port-serial.c:1309] _close_internal(): (ttyMux1) device open count is 1 (close)

The SIM in question doesn't have its number stored on it; I tried one which does and that one intializes ok.