Updates a SIP account by its id (display token). Only the fields sent are changed; omitted fields keep their current values, including the authentication method, IPs and the assigned Dynamic Caller ID rules. When dynamic_caller_ids is sent it replaces the assigned rules (same format as on create). Sending password (and optionally username) changes the SIP credentials; sending jwt_signature_secret replaces the JWT signature secret. Accounts provisioned by CommPeak (editable false) only allow changing the name, channel limit, tech prefix, features and Caller ID rules. Returns 422 with errors when the update is rejected.