ModemManager requires udev-powered Linux kernels in order to get notified of possible available Modems. udev will report each of the ports found in the device, and ModemManager will probe each of the ports marked with the ID_MM_CANDIDATE tag in udev. This includes both "tty" and "net" ports.
Aditionally, users of RS232-based devices may need to request additional manual scans via DBus, in order to detect modems that may have been connected to RS232 to USB adapters. In this case, udev just knows about the USB adapter being connected, not about the RS232 modem connected to the adapter, if any.