If you are trying to subscribe to a presence webhook and are receiving the following error:

 {
     "filter":"/restapi/v1.0/account/315352004/extension/315352004
         /presence/line/presence?detailedTelephonyState=true", 
     "reason":"SUB-412",
     "message":"Extension permission [hud] is missing"
 } 

Well, the good news is that there is a solution. The bad news is that only a RingCentral support representative is able to help you to resolve it. Reach out to support, and ask for help with "increasing the maximum number of monitored extensions."