I'm afraid you can't do that.. you cannot create your own softkeys in the phone context.
There are two other solutions:
1) Put a service on a line button
2) Create your own idle page which loads as soon as the phone is idle. You can then add your own softkey as you can with any application - however, since the application overlaps the labels of your line keys, you either lose those labels or you have to write the service in a way that it recuperates all line button labels and creates an image that looks like the real thing (you'll find an example on how to do that in the ip phone services sdk).
And just to make things really interesting, you cannot use the ?device=#DEVICENAME# syntax for an idle url so you have to look up the device from the IP address from which the request to your application comes.. which is doable (AXL Serviceability), but it'll take time to get it all done.