I'm wondering if it is possible to dial out a SIP URI from iOS Callkit. I can not find anything about this and what I tried does not work.
Using any SIP Library, you can make the call but using SIP URI alone, you can not make any call using callkit.
From apple doc
"CallKit lets you integrate your calling services with other call-related apps on the system. CallKit provides the calling interface, and you handle the back-end communication with your VoIP service. For incoming and outgoing calls, CallKit displays the same interfaces as the Phone app, giving your app a more native look and feel"
For all the sip functions, you have to call the respective callkit functions to communicate with the native phone UI. For ex.: if the caller disconnects the call, you will call sip_hangup function, at the same time you have to call the disconnect function of the callkit as well.