Phone validation glossary
Plain-English definitions for the phone number, carrier, and SMS terms that come up when you are cleaning a contact list or debugging deliverability.
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10DLC
The US carrier framework for sending registered application-to-person SMS from a 10-digit local number.
A
A2P messaging
Application-to-person SMS — messages sent by software to a person, rather than between two people.
C
Caller ID spoofing
Falsifying the originating number displayed on a recipient caller ID.
Carrier lookup
Querying network data to find which carrier currently serves a phone number.
CNAM (Caller ID Name)
The caller ID name database that supplies the text shown next to an incoming phone number.
D
DID (Direct Inward Dialing)
A phone number that routes directly to a specific extension or user, bypassing a receptionist.
Disconnected number
A number that is no longer assigned to an active subscriber and cannot be reached.
DNC list
A registry of phone numbers that must not be contacted for marketing purposes.
E
E.164
The international standard format for phone numbers: +, country code, then the national number.
H
HLR lookup
A query against a mobile network Home Location Register to check whether a number is active and reachable.
L
Line type
The classification of a phone number as mobile, landline, VOIP, or invalid.
List hygiene
Keeping a contact list accurate and current by removing or flagging unusable records.
M
MVNO
A mobile operator that resells service over another carrier physical network.
N
NPA-NXX
The first six digits of a North American number: the area code plus the three-digit exchange prefix.
Number portability
The ability to keep a phone number when switching from one carrier to another.
O
Opt-in
A recipient explicitly granting permission to be contacted by call or text.
P
PSTN
The Public Switched Telephone Network — the traditional circuit-switched phone system.
R
Reassigned number
A number reissued to a new subscriber after the previous holder gave it up.
S
Short code
A five- or six-digit number leased for high-volume, carrier-approved SMS campaigns.
SIM swap
Moving a phone number onto a different SIM card, legitimately or as an account-takeover attack.
SIP trunk
A virtual connection that carries voice calls over the internet instead of physical phone lines.
SMS deliverability
The proportion of sent SMS messages that are actually delivered to a recipient handset.
Soft bounce
A temporary message delivery failure that may succeed on a later retry.
T
TCPA
The US Telephone Consumer Protection Act, which governs automated calls and text messages.
Toll-free number
A number, such as an 800 or 888 prefix, where the recipient rather than the caller pays for the call.