RPC Error in AnswerPhoneNot An Issue
Players getting RPC kicks on AnswerPhone
Same error for my players
Are you using any plugins that do anything with phones?
No plugins for phones, its an Oxide issue. 
Make sure you have the latest rust and oxide versions. Mixing versions could result in errors.
Oxide is not related to this error, we do not hook it or anything. The only issue we have seen related to this is if your server is not fully updated and is using mixed Rust versions. Mixing Rust versions can happen if you install and ok'd Oxide build or do not update your server fully with Rust.
Right, still doesn't appear to be related to Oxide itself though.

Got the same problem, both my servers are fully updated and Oxide is updated on both aswell (I double checked).

NullReferenceException: Object reference not set to an instance of an object
TheLastBaron[PT] kicked: RPC Error in AnswerPhone

im using outpost phone with no kick error so all is good 
Yes im on a modded server
Locked automatically