So, there is an app in the store called "Ultimate Rotation Control," but you're probably better off using an aftermarket dialer if you frequently need to dial by manually entering numbers.
If you go into the "Per App Settings" and set the "Phone" app to "Forced auto," it will rotate the dialer when in landscape. However, the big caveat is that there isn't a native landscape view for the dialing screen (the numbers). Call log and Frequently Contacted work fine. If you switch over to dial by digits when in landscape, there are no numbers visible to press. Instead, there is a very faintly defined field just below the blue line where you have to tap to set an invisible cursor, and this brings up the keyboard in digit mode. Enter the number and press "Done" to dial. It's less than ideal, but it works.
If the only app you want to auto-rotate is the dialer (and not, say, the Home and Lock screens), then disable the global "forced auto" setting in Rotation Control and just set the per app setting for the dialer.