I'm actually working on the same problem and have a few posts about this in the Newbie area. It appears you need to do some manual tweaking of config files on the Asterisk side. To me, that's all clear. Take a look at this post where you modify sla.conf, zapata.conf and sip.conf.
The problem I'm having is on the Polycom side. For 3 trunks, there's 4 extensions which determine what Asterisk does with the incoming request. There's one for each trunk (for the line button they press), and one for when they just pick up the handset without selecting a line button. I'm not sure how to add that to the Polycom phone configuration.