In ObiTalk expert mode, you have to clear 2 boxes before entering your new value: OBiTALK Settings and then Device Default. Then you have to click Submit on the bottom of the same page before leaving that page.
The article that drgeoff explains that I expect, but mainly explainswhy you want to make the entries in ObiTalk.
There are a few things you can do only on the local OBi web server. The most common of these for me is to look at Status>Call Status.