I'm trying to use both the headset and handset simultaneously with the M10, M12, and M22, but I'm having problems. Should I be able to use both at the same time?
You cannot use both the handset and the headset simultaneously, but you can switch between the two.
If you are on the handset and wish to switch to the headset, place the handset aside (do not hang it up) and press the headset button on the amplifier. Or, if you are already on the headset and want to switch to the handset, pick up the handset and press the headset button on the amplifier to turn the headset off.