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It looks like there's a problem with the BM818's handling of the AT+CCLK? command. Other commands like AT+CFUN=? and AT+CREG=? work fine with carriage return characters but AT+CCLK? seems to require a variable number of line feed (and/or possibly carriage return) characters after, seemingly depending on its mood. This can result in the following under ModemManager:

ModemManager[1025]: <debug> [1574763430.732691] (ttyUSB1) device open count is 2 (open)
ModemManager[1025]: <debug> [1574763430.733302] (ttyUSB1): --> 'AT+CCLK?<CR>'
ModemManager[1025]: <debug> [1574763433.730930] Couldn't load network timezone: Serial command timed out
ModemManager[1025]: <warn>  [1574763433.731237] Couldn't load network timezone from the current network
ModemManager[1025]: <debug> [1574763433.731317] (ttyUSB1) device open count is 1 (close)
ModemManager[1025]: <warn>  [1574763433.731555] (tty/ttyUSB1) at port timed out 2 consecutive times
ModemManager[1025]: <debug> [1574763447.163110] Added CALL at '/org/freedesktop/ModemManager1/Call/0'
ModemManager[1025]: <info>  [1574763447.192843] user request to start call
ModemManager[1025]: <info>  [1574763447.192990] Call state changed: unknown -> dialing (outgoing-started)
ModemManager[1025]: <debug> [1574763447.193033] Setting up in-call unsolicited events...
ModemManager[1025]: <debug> [1574763447.193409] (ttyUSB1) device open count is 2 (open)
ModemManager[1025]: <debug> [1574763447.193748] (ttyUSB1): --> 'ATD07763578094;<CR>'
ModemManager[1025]: <debug> [1574763447.485991] (ttyUSB1): <-- '<CR><LF>+CCLK: "80/01/06,00:04:11"<CR><LF><CR><LF>OK<CR><LF>'
ModemManager[1025]: <warn>  [1574763447.487250] Couldn't start call : 'Couldn't start the call: Unhandled response '+CCLK: "80/01/06,00:04:11"''

I've tested AT+CFUN=? and AT+CREG=? using the following:

echo -ne 'AT+CFUN=?\r' > script
echo -ne 'AT+CREG=?\r' > script

and then sending the script file using 'C-A y' in minicom. I immediately get:

+CFUN: (0-1,4-7),(0-1)

OK

and

+CREG: (0-2)

OK

If I do the same with AT+CCLK?:

echo -ne 'AT+CCLK?\r' > script

then I get no response from the modem until I press enter in minicom.

I also checked the AT+CMER? command which also has no '=' symbol. It displays behaviour identical to AT+CCLK?. It looks like there is a problem with the command parser in the firmware on the BM818.