Greetings. I know this is an old post but I hope someone will benefit from my findings.
The menu to set up SIP services up is under the dialer setup menu and not under the general settings menu. (rant omitted)
First you need to set up your SIP server and password. Once that is done, toggle the "Receive Incoming Calls" setting and you will notice a server status message under the SIP Accounts. If you are registered to a SIP server it will say something like "primary account receiving calls".
Three options for "Use Internet Calling" are:
1 - "For all calls" - great if you have a solid data connection at all times
2 - "For Internet Calls Only" - you need to set up a contact in your contacts with routing information
3 - "Ask For Every Call" - this prompts you every time you dial.
Option 3 interferes with handsfree Bluetooth operation in a car. The BT equipped car can't get past the call routing dialog box asking if it is a voice or internet-call so the call fails.
Call quality is very good as long as there is a good data or WiFi connection.
As far as I can tell using the "For Internet Call Only" forces you to prepare a contact for each number. The contact then has the routing information.
Create a contact and use the "Add Another Field" pull down menu and select "Internet Call" then enter the extension or number you wish to dial.
I don't see any option to directly dial internet calls with the "for internet calls only" option set. This is my preferred way to use this feature creating a contact for each extension in the office and my desk phone rings at the same time as cell phone.
IMO they missed the 4th option "use internet calling if NO mobile net is available" so when there's no mobile data but WiFi available the calls could be routed that way automatically.
Note: this works very well with Trixbox type setups.