Modem manager gui is an abandoned project. You should not ruin your system by selectively downgrading other stuff to make it work.
There is an AUR project modem-manager-gui-hg that takes the now 4 year old source and patches it, there is of course no guarantee how long will this still work, since the original code relies on old libraries versions…and it will break again at some point in the future.
Alternatively, you can use other tools to achieve the same tasks. Network manager already supports modems out of the box. Gnome calls can be used for calls, if supported by sim and modem. Modem manager itself can send and receive sms from the command line. For USSD codes like for checking the prepaid balance, there is the option of a gnome prepaid manager applet if you use gnome or you can use a com program to send AT commands to modem and respectively ussd commands.
There are also packages like Wammu or Gammu for mobile devices, that are somewhat all-in-one.