If you’re running a hosted PBX solution with HPBX phones deployed at customer sites, there’s a chance you’re unknowingly generating unnecessary “noise” on your network.
What kind of noise, you ask? The kind that stems from a configuration mismatch — and yes, it can cause problems.
Unsolicited Notifications vs. SUBSCRIBE Messages
Phones that use Line State Monitoring or Message Waiting Indication (MWI) for voicemail can be configured in two main ways:
- Unsolicited Notifications
In this setup, the phone doesn’t request updates. Instead, the switch automatically sends a SIP NOTIFY message whenever there’s a new voicemail or when the mailbox is cleared (or a change to the line state being monitored). - SUBSCRIBE Messages
As the name suggests, this configuration requires the phone to send a SUBSCRIBE message to the switch, along with an expiry. The phone must periodically renew its subscription to continue receiving updates.
Why a Mismatch Matters
Problems arise when the switch is set up for Unsolicited Notifications, but the phone is configured to use SUBSCRIBE messages. Here’s what happens:
- The phone sends a SUBSCRIBE message.
- The switch responds with a
401 Unauthorized, asking for authentication (via NONCE). - The phone resends the SUBSCRIBE with the requested NONCE.
- The switch replies with a
405 Not Accepted Here.
This signaling loop repeats every few seconds — endlessly. If you have a significant number of misconfigured HPBX phones, this can generate tens or even hundreds of thousands of SIP messages per hour. That’s the “noise” we’re talking about.
The Impact of SIP Noise
All these unnecessary SIP messages consume critical resources:
- Switch and SBC processing power: Every SIP message must be handled, which strains CPU and memory resources on devices like the Perimeta SBC and the switch (CFS).
- Disk space: SAS logs every interaction, rapidly filling storage capacity.
- Rate limiting and bans: The Perimeta may start rate-limiting SIP messages from specific IP addresses or even block them altogether. This could result in legitimate SIP messages (like new calls) failing.
In the worst-case scenario, this misconfiguration could lead to a service outage.
How to Detect the Problem
To find out if your network is at risk:
- Check telemetry: Use tools like MetaView Explorer Statistics, Perimeta statistics, Cacti, SolarWinds, or Nagios to monitor SUBSCRIBE message trends and failures.
- Analyze SAS data: Look for
405 Not Accepted Hereresponses or run reports to identify SIP lines generating the most call traces.
These insights will help you determine if misconfigured phones are contributing to network noise.
How to Fix It
If you suspect or confirm that you’re affected:
- Update configurations: The key setting is a fix bit on the Remote Media Gateway model called “Expects unsolicited notifications”. If that’s set to True, then the CFS will send those notifications whether requested or not AND it will reject any attempts to SUBSCRIBE. If it’s set to false, then the phone has to SUBSCRIBE to the information it wants to receive. Each phone model is different (and in some cases you can configure the phones for one behavior or the other), but as long as your fix bit matches the phone’s behavior you should be fine.
- Reach out for a deployment review: If you’re an existing client, we would be happy to assess your current setup, identify misconfigured devices and work through the process of fixing it. If we don’t currently work with you, we’d be happy to review this issue along with others as part of our system audit service.
Fixing this issue can be time-consuming, but it’s essential for maintaining a healthy network. And you don’t have to tackle it alone — we’re here to help.
Don’t Let Noise Disrupt Your Network
The longer this issue goes unresolved, the greater the potential for resource strain or service impact. We have seen service outages where this issue was a significant contributing factor. Contact us today to review your deployment, reduce unnecessary noise, and protect your network from preventable outages.
