Just a quick question.... Has anyone tried to make BLF's work with phones registered directly to 2Talk?

I am just testing this out with a couple of SPA504G's.
I have used a similar approach to doing this as you would do it with Asterisk.
The phones have their 'extension' registered as line 1 they have their line appearance set as 'shared'. Problem is that won't register properly with 2Talk. When put back to private they are good as gold.
Other setting is the 'Line Key Extended function' for the keys I want to use for BLF's, I have entered the following in to them...
fnc=blf+cp;sub=028255XXXXX@27.111.14.65
Just doesn't seem to work....
Have also tried
fnc=blf+sd+cp;sub=028255XXXXX@27.111.14.65;ext=028255XXXXX@27.111.14.65
fnc=blf+sd+cp;sub=028255XXXXX@27.111.14.65;usr=028255XXXXX@27.111.14.65

Can anyone tell me other options are available to put in the function keys? Is it possible that I need to send a password? Or should it just poll the 2Talk proxy to see what that lines is doing regardless?

Any help much appreciated.

Cheers,
Sam