Can you use Windows TEL Protocol to call out from a desk phone? With the Ringcentral app you can! Since it works with Ringcentral other phone services/apps may have the same feature, contact you phone service provider to see what features they support.
With Ringcentral the Windows phone app has this in settings. If you choose "Direct dial" when calling out it will call from the app, if you change it to "RingOut" and choose your desk phone it will initiate the call with your desk phone. You have the option to prompt you to dial 1 before connecting or not.
Note: The only support we provide for setting up your phone system for use with RTO Pro are the instructions in these help topics. Any other information/details would be provided by your phone service provider. The only system we have tested the Call Out feature using Windows TEL Protocol is RingCentral. Contact your phone service provider to find out if your service is compatible with this feature.