SIP horns: What They Are and How They Affect VoIP Call Quality
When people say SIP horns, a term used in VoIP to describe malformed or overloaded SIP registration attempts that flood a server with invalid requests. Also known as SIP registration storms, it's not a hardware problem—it's a network attack that looks like a storm of failed login attempts from bots or misconfigured devices. Think of it like someone banging on every door in an apartment building at once, trying every possible key. Your VoIP server gets overwhelmed, legitimate calls drop, and suddenly your phone system is dead—even though the internet is fine.
SIP horns usually happen when attackers scan for open SIP ports and try to guess passwords using automated tools. They don’t care if they break your system—they just want to hijack it for toll fraud. But they can also come from inside your network: a misconfigured IP phone, a faulty ATA, or a device stuck in a retry loop after a password change. These aren’t just annoying—they’re dangerous. A single SIP horn can drain your bandwidth, crash your PBX, and open the door to thousands of dollars in unauthorized international calls.
Fixing SIP horns isn’t about buying fancier gear. It’s about locking down your system. SIP registration, the process where VoIP devices authenticate with a server to receive incoming calls. Also known as SIP authentication, it’s the first line of defense. Use strong passwords, disable MD5 digest auth, and enable TLS encryption. Set up rate limiting so no device can send more than 5 registration attempts per minute. Tools like Fail2ban can auto-block IPs that behave badly. And don’t leave SIP ports open to the public internet—use a Session Border Controller (SBC) to filter traffic before it even hits your server.
What you’ll find in the posts below isn’t theory—it’s what actually works. Real setups from businesses that stopped SIP horns cold. You’ll see how to spot the signs before your phone system goes silent, how to configure your ATA or IP phone to avoid accidental storms, and which VoIP providers actually block these attacks out of the box. No fluff. No jargon. Just clear fixes that keep your calls ringing.
