I gave up on the BLF as the Presence/Switchboard is more functional. Right-click the extension/call (not BLF) and "Take" or dial *20*ext. You should also make sure you have the appropriate group rights enabled (Extensions>*Group>Member Rights).
It does work. You have to use the "Custom Speed Dial" option and then the number should be "*20*" to pick up any call in the call group. I have implemented this in my solutions where required.