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What is the maximum number of simultaneous SIP registrations is allowed?

Flowroute restricts a single user account to 24 simultaneous SIP registrations. This is a high-level threshold for SIP registrations.  Beyond 24 SIP registrations identify a need to explore a better process. We will discuss here, some options and define the reason for SIP registrations. 

The SIP registrations are on the Interconnection > Status section. You may see any SIP registrations on the account.

NOTE: The SIP registrations on the list may be delayed slightly when a connection is ended.


SIP registrations are like a trail of crumbs from your PBX to your SIP provider’s servers. When you’re using SIP registrations, an inbound call INVITE can follow the registration trail back so it knows where to find the PBX within your local network.

Routing calls based on SIP registration will send an INVITE to every actively-registered SIP device on your account. That means if you have PBX's at two locations, and both are registered to the same SIP account, both PBXes will receive an INVITE to the call. This creates a “race condition” in which the first PBX that sends a SIP response can dictate the ultimate behavior of the call – even if a subsequent SIP response issues a conflicting direction.

With host-based routing, you can specifically designate which PBX inbound calls route to by directing a phone number to a particular PBX’s IP or domain name. You get more control of where calls go, and this helps prevent unintended behavior such as calls misrouting or dropping instantly.

Host-based routing requires a static IP address or a domain name. If your ISP provides you with dynamic IPs and you don’t already have a domain name set up, you can create a fixed host domain using dynamic IP address services like DynDNS, and no-IP. These types of services communicate with your router to associate each new IP address with a static domain. You can then use your static domain as your host route.

Once you have a static IP or domain to route inbound calls to, your provider can send calls to your PBX without requiring a REGISTER request because they already know where to send the calls.

The restriction of 24 registrations is helpful as a reminder that SIP registration is not always the answer for PBX uses. 

If you are seeing you have more registrations than are allowable, you will need to explore Host-Based Routing. As read above, the host-based routing is more effective for larger numbers of PBX connections. 

Registration is not needed for inbound calls; there is more than one option for receiving inbound calls. 

If you need help with understanding how to better manage your PBX registrations feel free to contact our support at