Dear Alessandro,
Thank you for your email.
This is expected behaviour. When you attach a Plantronics headset/amplifier like the DA45 to the MDA200, the MDA200 acts as a hub/pass-thru, so the Plantronics SDK will only report the DA45.
The button features of the MDA200 are not programmable via the SDK. The MDA200 is designed as a UC enabler device. If you need finer grained control to activate lines, hold/resume lines etc, you should look at our Savi 700 range of products which do support programmatic line control via the Plantronics SDK.
Thanks,
Lewis.