Mobile, landline or VoIP: why line type decides your bounce rate

Line type is the field that decides whether an SMS campaign works. Send to a landline and the message is accepted, billed and thrown away — no bounce, no error, no signal in your reporting that anything went wrong. It just quietly does not arrive.

Worth understanding what the field actually means, because it is less absolute than it looks.

The three types

Mobile

A number assigned to a cellular network. Receives SMS. This is the segment almost every text campaign is actually aimed at.

Landline (fixed line)

A number assigned to a fixed-line exchange. Cannot receive SMS in any useful sense. Still perfectly good for a voice campaign — a landline is not a dead number, it is a number for a different channel.

VoIP

A number assigned to a voice-over-IP provider — Twilio, Google Voice, a cloud PBX, a business phone system. SMS delivery is inconsistent and depends entirely on the provider. This bucket contains both serious business phone systems and disposable throwaway numbers, which is why a blanket rule for it is always wrong.

How line type is determined

Two different mechanisms, depending on the country, and the difference explains most confusion about accuracy.

Where the plan separates them (most of the world)

Most numbering plans reserve distinct ranges for mobile and fixed line. The line type is readable from the number itself:

• UK — +447… is mobile. +4420…, +44161… and similar are geographic fixed lines.
• Nigeria — +23470…, +23480…, +23481…, +23490…, +23491… are mobile. +2341… is Lagos fixed.
• Germany — +4915x, +4916x, +4917x are mobile; geographic codes are fixed.
• India — mobile numbers are ten digits starting 6 to 9.

Here line type is as reliable as the plan data itself. It does not depend on knowing anything about the individual subscriber.

Where they share the space (the NANP)

The United States, Canada and the Caribbean territories on +1 do not separate mobile from fixed by range. A 415 number can be either. Line type there comes from the assignment record of the specific NPA-NXX block — which carrier it was allocated to, and whether that carrier is a wireless or wireline operator.

The ported-number limitation, stated plainly

Since 2003, US subscribers have been able to keep their number when changing carrier. Local number portability means the block's original assignment and the number's current carrier can differ.

So a block-based lookup tells you what a number was allocated as, not necessarily what it is today. A landline number ported to a mobile carrier will still read as landline. In practice this affects a minority of numbers, and it affects line type and carrier — it does not affect whether the number is allocated at all, which stays deterministic.

What to do with each type

Note that landlines move rather than get deleted. A landline is a working number and a legitimate contact — it is only useless for the channel you happened to be looking at. Deleting them is throwing away a voice list you already paid to build.

Why the bounce rate matters beyond the wasted send

Carriers score senders. A high proportion of undeliverable messages looks like the traffic pattern of a spammer working through a bought list, because that is usually what it is. The consequences escalate: throughput throttling first, then filtering, then a blocked sender ID.

Cleaning the list is not only about the money you save on this campaign. It is about the next campaign still being deliverable.