VX8550 speakerphone

With slide open, pressing and hold the hot key for the speaker turns on the speaker, with a warning that it will turn off in a minute. All is well with this functionality.

What I can't figure out is... if I answer a call, with the slide open how do I then activate the speaker. The usual hot button is not lit up at this point, in order to activate the speaker.

I have searched all over but don't seem to be able to find a solution.

Many thanks!!

Update - with the slide open, the phone is technically unlocked. While on a call if you press the unlock slide button on the right side of the phone down, the 4 'hot' keys on the phone will light up, then press the speaker button and it will turn on the speaker.