BLF Works but not as expected

Hi Guys,
The Yealink phones work brilliantly except for one issue I would like to resolve.
I have set up BLF on the phones and when I am on the phone my identifying light is RED, when I receive a second call in the background which then goes through to someone else to be answered it then knocks out my light (back to GREEN), is this a setting I can change? my SIP line supplier do not seem to be able to help as they do not support Yealink phones, but BLF feature is fully supported apparently.
Thanks in advance.
Dave