It is possible to operate the A190 headset as a stand alone device or to pair the headset with an handset registered at the base station. This works for all the handsets and base stations on the M-series, so M25, M65, M70, M80, M85 and M90 working on M700 and M900 (for the M300 there are some additional steps to be taken, reported at the bottom of this guide).
The headset will get the same extension number as the handset.
On the "extension" page, click on the IPEI of the the A190, the A190 handset page will open and you will be able to pair the devices
In the pull-down list of "Paired Terminal", select the "Terminal Idx" of the handset.
When you click on "Save", the headset is now registered with the same extension number as the handset
To make this work on an M300 you would need to disable the "Local Call Groups" option, please follow these steps
Now you should be able to follow the above steps and pair the A190 with any handset you need on the M300.