Say Hello to SIP Code 603+: The FCC’s New Standard for Analytics-based Blocked Calls

If you’ve ever tried to make a call in the U.S. and it didn’t go through, you might have seen a little breadcrumb left behind: a SIP error code. These codes are like the network’s way of explaining what happened. 

For years, blocked calls could come back with a handful of different codes (603, 607, or 608) depending on the situation. Each one carried a slightly different meaning, but for developers and ops teams, it often boiled down to the same question: “Why didn’t this call complete?”

The Federal Communications Commission (FCC) has decided it’s time to enhance the feedback loop for originators making calls. And that’s where SIP code 603+ “Network Blocked” comes in.

Out with the old, in with 603+ 

The FCC recently issued an order that requires carriers and voice providers to use 603+ as the single, uniform way to indicate that a call was blocked by analytics on IP-based networks.

In plain English: instead of juggling three different codes, everyone will now use one for analytics-based blocking. The goal is consistency, so no matter which carrier your calls travel across in the U.S., you’ll see the same signal if they’re blocked based on reasonable analytics. The added benefit of SIP 603+ allows callers to more readily access redress with the operator when legitimate calls are blocked.

When 603+ makes its entrance

Carriers can begin adopting 603+ right away, and many will roll it out gradually. By March 25, 2026, though, its use will be required across the industry. That means over the next year and a half, you may start to see 603+ appear alongside (or instead of) the older codes, until 603/607/608 are fully phased out.

As carriers make the switch, here’s how the change will show up across Twilio products:

• Elastic SIP Trunking (ESIPT): When a carrier partner returns a 603+ on a Termination call, Twilio will pass that response to your SIP endpoint.

• Programmable Voice SIP (PV SIP): If inbound calls to Twilio SIP Domains are immediately forwarded to the PSTN with the answerOnBridge attribute set to "true", and the outbound/child call receives a 603+, Twilio will pass that code back to your SIP endpoint on the parent/inbound call.

• Bring Your Own Carrier (BYOC) Trunking: In scenarios where BYOC traffic follows the same forwarding path described above, a 603+ from the carrier will be returned to your SIP endpoint on the parent/inbound call.

• Programmable Voice (non-SIP): No impact here, since SIP responses don’t flow through these call types.

It’s also worth noting that this new code only applies to calls blocked by analytics (like suspected spam). Other types of blocking, such as “Do Not Originate” (DNO) lists, Commission-directed blocking, or customer-requested blocks, aren’t affected.

Do you need to do anything? 

For most customers, no action is needed. The only thing to keep in mind is that if your team’s systems or logs specifically look for SIP codes 603, 607, or 608, you’ll want to update them to handle 603+ as carriers begin using the new standard.

The bottom line: 603+ is here to make analytics-based blocked call signaling simpler, clearer, and more consistent. Just be aware that as U.S. carriers adopt the new standard, you’ll start to see it in your call flows.

And as always, thank you for building with Twilio!

–Team Twilio