SIP 401 Unauthorized: What It Means and How to Fix It
When you see SIP 401 Unauthorized, a standard SIP response code indicating the client failed to authenticate properly. Also known as SIP 401 error, it’s one of the most common reasons your VoIP calls won’t connect — even when your internet is fine. This isn’t a network issue. It’s not a dropped signal. It’s a security gate shutting you out because your credentials don’t match what the server expects.
This error happens at the SIP authentication, the process where your device proves its identity to a VoIP server using username, password, and sometimes digital certificates. Think of it like trying to log into your email with the wrong password. The server says, ‘I know who you’re supposed to be, but you’re not proving it.’ That’s SIP 401. It’s not random. It’s intentional. And it’s fixable. Most people blame their router or ISP, but the real culprit is usually a misconfigured SIP account, an expired password, or a firewall blocking the authentication handshake.
The fix starts with checking your SIP client settings. Did you enter the correct username and realm? Is the password still valid? Many VoIP providers reset passwords after 90 days — and if your phone or softphone hasn’t been updated, it keeps trying with the old one. Also, check if your device is using the right authentication method. Some servers require MD5 hashing; others need plain text. Mixing them up triggers SIP 401 every time. If you’re using a business system, make sure your user account hasn’t been disabled or moved to a different trunk. And don’t overlook VoIP security, the layer of protections that includes firewall rules, SIP ALG settings, and encrypted transport. Some routers have SIP ALG turned on by default, which messes with SIP headers and breaks authentication. Turning it off often solves the problem.
There’s also a hidden layer: SIP setup, the configuration process that ties your device to your provider’s network. If you’re using a hardware phone, did you flash the latest firmware? Outdated firmware can send malformed authentication requests. If you’re using a softphone like Zoiper or Linphone, check if your account profile is saved correctly — sometimes importing a backup file overwrites the right settings with wrong ones. And if you’ve switched providers recently, your old SIP credentials might still be lingering in the device’s memory.
You’ll find plenty of posts below that dig into these fixes — from step-by-step guides on reconfiguring SIP accounts to troubleshooting firewall conflicts that block authentication. Some show how to test your SIP registration with command-line tools. Others explain how to read SIP logs to spot exactly where the 401 is triggered. There are also deep dives into why some providers use digest authentication while others don’t, and how to avoid common mistakes when setting up new VoIP lines. Whether you’re a small business owner, a remote worker, or just tired of hearing ‘call failed,’ these posts give you the real fixes — not guesswork.
