This bug is related to the following:

https://bugs.freedesktop.org/show_bug.cgi?id=89468

Version info:

jh@ubuntu:~$ apt-cache policy network-manager
network-manager:
  Installed: 0.9.8.8-0ubuntu7
  Candidate: 0.9.8.8-0ubuntu7
  Version table:
 *** 0.9.8.8-0ubuntu7 0
        500 http://us.archive.ubuntu.com/ubuntu/ trusty/main i386 Packages
        100 /var/lib/dpkg/status
jh@ubuntu:~$ apt-cache policy modem
modem-cmd          modemmanager       modemmanager-dbg   modemmanager-dev   modemmanager-doc   modem-manager-gui
jh@ubuntu:~$ apt-cache policy modemmanager
modemmanager:
  Installed: 1.0.0-2ubuntu1
  Candidate: 1.0.0-2ubuntu1
  Version table:
 *** 1.0.0-2ubuntu1 0
        500 http://us.archive.ubuntu.com/ubuntu/ trusty/main i386 Packages
        100 /var/lib/dpkg/status

Copy from other bug report:

I'm on Ubuntu 14.04 on a 32-bit machine and using ModemManager 1.4. I've also tested the following on ModemManager 1.0 with the same results. I'm in a scenario where I have two MC7304 Sierra modems, both connected to ModemManager through USB.

When I issue 'mmcli -L' I get the following:

	/org/freedesktop/ModemManager1/Modem/1 [Sierra Wireless, Incorporated] MC7304
	/org/freedesktop/ModemManager1/Modem/0 [Sierra Wireless, Incorporated] MC7304

I then enable both modems. I do this by issuing the commands:

mmcli -m 0 -e
mmcli -m 1 -e

Both end up in state 'registered'. So far so good...

           |          state: 'registered'

However, when I disable one of the two modems, and only one, the other one disables as well. If I issue the command:

mmcli -m 0 -d

both modems will end up in:

           |          state: 'disabled'

It seems that disabling one modem disables the lot of them. The same principle applies for 3 or 4 modems which I have also tested.

After investigation from Aleksander Morgado (ModemManager), it seems that NetworkManager is causing the second modem to disable for some reason. I will include the logs as attachment.