On March 25, 2026, the FCC‘s Eighth Call Blocking Order introduced a new requirement for U.S. carriers to use a response code called SIP 603+. It is a meaningful shift in how blocked calls are reported, and it gives businesses something they never had before: a clear, standardized signal every time a call is blocked by carrier screening.
A little background
Since September 2025, blocked calls have become a bigger and bigger problem for teams making outbound calls. Several major shifts landed at the same time, and the combination is what made contactability harder to navigate. Carriers got stricter about which calls they let through, and consumers became harder to reach on unknown numbers. New FCC registration and labeling rules added more steps for businesses. And both Apple and Android updated their phones to screen calls more aggressively, flagging more numbers as “Spam Likely.”
We have been talking about this with our customers for a while now, sharing tools and resources to help. The 603+ update is the newest piece of this puzzle, and it is worth understanding on its own.
What is SIP 603+?
Before this rule, when a carrier blocked a call, your dialing system had no way to know what happened because it just looked like a failed call. Your system might flag the number as bad, your team moves on, and no one realizes the real issue is that the carrier is actively blocking your outbound number.
SIP 603+ fixes that. It is a specific signal that carriers now send back when they block a call based on spam or fraud detection. It tells your system: “This call was blocked. Not a bad number. Not a connection problem. A carrier made a decision to block it.”
The older codes, 603, 607, and 608, are being retired for this purpose. 603+ is the new standard.
Think of it like an email bounce code. Instead of just knowing a message did not go through, you now get a specific reason why. That makes it much easier to take action.
Why this matters for your team
If your dialing platform has been updated to read 603+, you now have something you did not have before: real-time visibility into when a carrier is blocking your calls. You do not have to wait weeks to notice a drop in your answer rates. You can see it happening and respond faster.
It also gives you better documentation if you need to go through a carrier’s dispute or remediation process to get a number unblocked.
But if your platform has not been updated to handle 603+ yet, none of this helps you. Your system will still read those blocked calls as generic failures, and you will keep troubleshooting the wrong thing.
What to do right now
Ask your dialing platform about 603+ support
The first step is simple: ask. Contact your telephony provider and find out if they have updated their system to recognize and log 603+ response codes. If they have not, request it. This is not a small matter to let sit.
Check your number reputation
Carrier labeling changes often. A number that was clean last month may be flagged today. If a number is marked as “Spam Likely,” that is a separate issue from 603+ but worth knowing at the same time.
Modernize offers a free number reputation checker that shows instantly whether your outbound numbers are flagged as spam, along with steps to address it.
Look at your outbound caller IDs
Here is a pattern worth watching for: if a lead seems undialable but goes through fine when you call from a different number, the problem is almost certainly your outbound caller ID, not the lead’s number. That points to a carrier-side block, and 603+ will now help you confirm it faster.
The bottom line
603+ does not solve the blocked calls problem. But it makes the problem visible in a way it never was before. The teams that adapt quickly, by making sure their platforms are reading the new code, will have a real advantage in diagnosing and fixing blocks before they drag down results.
We will keep sharing updates as carriers roll this out more broadly. If you have questions or want to talk through what you are seeing on your end, reach out. This is exactly the kind of thing we are tracking closely.
