Number reachability

Number reachability is whether a phone number can receive a call or message right now: it is assigned to a subscriber, the subscriber's phone is attached to a network, and the network accepts traffic for it. It is a stronger property than validity, which only says that a number is correctly formatted and fits its country's numbering plan.

What are the levels of checking a number?

It helps to think of four levels, each answering a different question:

Each level catches problems the one before it misses. A number can be perfectly valid and never have been assigned. A mobile number can be assigned but switched off. A reachable number might have no WhatsApp account.

Why isn't a valid number enough?

Validation only checks the digits. It catches typos and impossible numbers for free, but it can't tell you whether a phone at the other end exists. Lists built from old forms, bought data or manual entry are full of numbers that pass validation but have since been disconnected, recycled or never existed. Sending to them wastes SMS fees and harms delivery statistics. Some messaging providers also slow down senders with high failure rates.

What does "reachable" mean in practice?

For mobile numbers, the home network's subscriber register knows whether the number is in service and whether the SIM is currently attached. An HLR lookup reports this as:

• reachable: in service and attached; messages should be delivered.
• unreachable: in service, but the phone is off, out of coverage or temporarily not attached.
• invalid: not assigned to any subscriber.

Reachability changes minute by minute. A phone switched off overnight is unreachable at 3 a.m. and reachable at 9 a.m. Treat one unreachable answer as "try later" and repeated ones over weeks as a sign of an abandoned number.

How does channel reachability differ?

Messaging apps have their own account databases. A number with a WhatsApp, Telegram or Viber account passed that app's own sign-up verification at some point. This is useful when you choose a channel for a message the user expects, and it is a sign that someone really used the number. It doesn't say the phone is on right now, and apps keep inactive accounts for a while before deleting them.

Which checks should I run?

It depends on what a failure costs you:

• Before every OTP: validity plus line type. Add a channel check if you offer app-based delivery.
• Before a bulk campaign people have agreed to receive: validity, line type and reachability, run as a bulk job.
• For lead verification: validity, line type, a channel check and spam reputation where available.

MobileValidate's HLR lookup returns live reachability (reachable, unreachable, invalid) for mobile numbers, with unknown answers free. Carrier, porting (MNP lookup), channel and spam checks are available too.