You may see an error message indicating a SIP protocol error 401. There are several steps you can take to resolve this issue. So we’ll be looking at it soon. SIP 401 – Unauthorized usually means that the request requires user authentication. This error message is returned by the SIP server. Only your PBX administrator or VoIP provider knows exactly why this response is being sent to the softphone. … Some VoIP servers refuse multiple registrations.
Wikipedia listing article
Session Initiation Protocol (SIP) is a signaling protocol for managing communication sessions such as IP phone calls. SIP is based on request / response transactions, similar to the Hypertext Transfer Protocol (HTTP). Each transaction consists of a SIP request (one of many request methods) and at least one response.  : page 11
SIP requests and responses can be generated by any SIP user agent. User agents are divided into clients (UAC) that initiate requests and servers (UAS) that respond to them.  : §8 A single user agent can act as both User Account Control and User Account Control for different transactions:  : p26 For example, a SIP phone is a user agent that controls the user account during a call. and UAS upon receipt. In addition, some devices act for both user account control and user account control for a single transaction. They are famousas concurrent user agents (B2BUA).  : page 20
SIP responses provide an integer 3-digit response code, which is one of many specific codes that detail the status of the request. These codes are grouped according to their first number, such as “try”, “success”, “redirect”, “client failure”, “server failure” or “global failure”, which corresponds to the first number from 1 to 6. For example , they are expressed in “1xx” for provisional responses with codes from 100 to 199.  : §7.2 SIP response codes correspond to HTTP response codes, although not all response codes HTTP are valid in SIP.  : §21
SIP replies also include a “base phrase” and a default base phrase is defined for each response code.  : §7.2 These basic sentences are subject to change, for example, to provide additional information  : § 21.4. 18 or provide text in another language.  : §20.3
SIP Response Codes and their principles were originally defined in RFC 3261.  This RFC also definesWrite the Internet Assigned Numbers Authority (IANA) for SIP parameters so that other RFCs can provide more response codes.  : §27 
This list contains all SIP Response Codes defined in the IETF RFCs and registered with the IANA Registry for SIP Settings as of July 14, 2017 [update] . This list also contains SIP Response Codes that are defined in legacy SIP RFCs (specifically RFC 2543) and therefore are not registered with the IANA. they are clearly marked as such.
What Is A Kingdom?
In general, SIP authentication is important for a specific domain, a security domain. Thus, each of these security domains has its own set of usernames and passwords for digest authentication.
Quote from RFC2617:
Method 405 Forbidden
Indicates that the request contains a list of prohibited methods.
Example: A REGISTER request can be sent to a user agent.
It contains an authorization field that informs UAC about valid methods.