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 as shown in How do I pair the A190 headset with an Mxxx series DECT handset.
Once the two devices, headset and handset, have been paired, they will share the same SIP extension and it will be possible to receive calls on both of them at the same time, or to start a call from each single device.
If after answering/starting a call you wish to switch between the two devices, you can follow the steps below.
When there is an incoming call on the shared SIP extension, you can answer the call using the A190 call button.
You can also start a call from the A190 if you set some buttons to do so, as shown in How to assign functions to the A190 buttons.
In both cases, you will be in a situation in which you are talking with somebody using the A190 itself.
To "steal" the call from the A190 and continue using any of the DECT handsets from the M-series, you can simply press the green receiver button on the handset, as shown in the screenshot below: now you can continue speaking using the handset (M25, M65, M70, M80, M85 or M90).
When there is an incoming call on the shared SIP extension, you can answer the call using the DECT handset.
You can also start a call from the handset, so that you can call any number as in an usual outgoing call.
In both cases, you will be in a situation in which you are talking with somebody using the DECT handset at the moment.
To "steal" the call from the handset and continue using the A190, you can simply press the call button on the headset, as shown in the screenshot below: now you can continue speaking using the A190.
Since you can configure the A190 buttons to perform different operations, it may be that you changed the call button functionality when it's pressed during a call, so stealing a call will not be possible anymore since it was re-configured.
If you want to go back to the "default" functionality of the call button while on a call, which is stealing the call from the handset, you can upload an MMI file where the "State: CallConnected -> Key: Hook-> KeyPress: Single" isn't defined.
For more information about MMI file, please take a look here How to assign functions to the A190 buttons
Also be aware that when paired, the A190 will not accept the definition of buttons via the MMI file: you would need to un-pair it, change the buttons as you wish, and then pair it again.