US Phone Verification Guide: +1 Numbers, Short Codes, 10DLC and Carrier Filtering
Everything that affects OTP delivery in the United States — number types, 10DLC registration, toll-free verification, short codes and how to test +1 signup flows.
OTP Bolt Team · · 8 min read
The United States looks simple — one country code, one numbering plan — but its SMS ecosystem has some of the strictest carrier filtering in the world. If your verification codes reach some American users and not others, the answer is usually in the rules below.
The North American Numbering Plan
US (and Canadian) numbers follow the NANP: +1, a three-digit area code, a three-digit exchange and a four-digit line number — +1 415 555 0134. There's no separate mobile prefix, so you can't tell a mobile from a landline by the digits alone. Many verification providers do a carrier lookup to decide whether a number can receive SMS.
Three ways to send A2P SMS
Application-to-person (A2P) messages — including OTPs — must use one of three sender types:
| Sender | What it is | Typical use |
|---|---|---|
| 10DLC | Ordinary 10-digit local numbers, registered for A2P | Most businesses |
| Toll-free | 8XX numbers, with toll-free verification | Customer care, OTP |
| Short code | 5–6 digit codes, leased and vetted | High-volume brands |
10DLC registration
Since 2023 the major carriers block or heavily throttle unregistered A2P traffic on local numbers. Registration goes through The Campaign Registry: you register your brand (legal entity, tax ID), then each campaign (use case — "two-factor authentication" is its own category) with sample messages. Throughput depends on your brand's trust score.
Toll-free verification
Toll-free numbers must be verified before sending A2P SMS. It's often faster to set up than 10DLC and works well for OTP traffic.
Short codes
The fastest, most reliable option, but also the most expensive and slowest to provision.
If your codes are silently disappearing for US users, the first question is always: is this sender registered for this use case?
Message content matters
Carriers inspect content. Keep OTP messages short and unambiguous:
Your Acme code is 482913. It expires in 5 minutes. Don't share it.
Avoid URL shorteners, ALL CAPS and anything that looks like marketing in a verification message.
Receiving codes: what testers need to know
On the receiving side, some American services refuse to send codes to numbers they classify as VoIP or non-fixed. That's a policy choice by the service, not a network failure. Two options cover almost every case:
- Temporary +1 numbers — rent one in OTP Bolt for a single verification. Many services accept them; live stock is shown per service before you rent.
- Dedicated +1 numbers — buy a US line under My Numbers and keep it for weeks or months. It gets a persistent SMS inbox and browser calling, and behaves like a stable line for repeat logins.
Canada in brief
Canada shares the +1 code and NANP format, but its carriers have their own registration programmes for A2P traffic. Bilingual (English/French) message templates are common; make sure your SMS fits within a single segment in both languages to avoid split messages.
A testing checklist for US flows
- Test with both a temporary and a dedicated +1 number — services treat them differently.
- Confirm your phone input accepts
(415) 555-0134,415-555-0134and+14155550134. - Verify your sender's 10DLC or toll-free registration covers the 2FA use case.
- Check delivery on more than one carrier if you can.
- Make sure your resend timer is at least 30 seconds; US filtering penalises bursty traffic.
Summary
US SMS delivery depends far more on registration and content than on the network itself. Register your sender for the right use case, keep messages clean, and test with real +1 numbers — temporary for breadth, dedicated for continuity.