We ran into the same .NET Framework compatibility issue as described in this post (https://forums.developers.ringcentral.com/questions/9674/how-to-add-ringcentral-nuget-for-net-452.html).
So we are using the legacy SDK found here (https://github.com/ringcentral/ringcentral-csharp).
I've read through the documentation found here (https://ringcentral-csharp.readthedocs.io/en/latest/).
But I have not been able to find anything about how to make a call out. Is there other documentation out there about the legacy SDK?