The MDM does not give your employer a way to retroactively unlock the phone. Depending on the MDM solution and capabilities they allowed they may be able to install an application though. But most people that have accepted MDM on their personal device from their employer, the only thing the employer can do is remotely wipe the device.
I don't think this is correct, Jamf has a "Clear Passcode" option that I have used with success, although it does require the device to have an internet connection.