Valid doesn't mean reachable: what to check before you treat a list as sendable

We spend a lot of effort on routing, termination quality and pricing - and comparatively little on the number list itself. But a list that "looks fine" is often the thing silently burning money.

The distinction that matters operationally:

- Syntax / format: numbers are E.164, no typos, right length. Important, but it only tells you the number is well-formed.

- Number in service / line type: the number is actually assigned, and whether it is mobile, landline, VoIP or a virtual/non-geographic range. This decides SMS feasibility and price class on many routes.

- Reachability / registration signals: whether the number is reachable on the channel you actually want to use (SMS, WhatsApp, Telegram...). A number can be perfectly valid and still not be a WhatsApp number, or not accept SMS.

- Freshness: a check from 11 months ago answers a different question than one from today. Ports, disconnects and channel eligibility change.

Practical workflow before a campaign:

1. Normalize everything to E.164 first. Dedupe after normalize, not before - "07123 ..." vs "+4471..." are the same number written two ways.

2. Run line-type classification; drop or reroute landlines from SMS lists.

3. If you are using a messaging app as the channel, check the registration signal for that app.

4. Preflight on the full list before you pay a provider for delivery, so you only pay to send to numbers that can actually receive.

We use PhoneVeriflo (https://phoneveriflo.com) for this because it does normalize + dedupe + line-type + per-channel registration signals in bulk, with an API we can throw routes at. But even if you build it in-house, run these checks - the cheapest send is the one you never had to make.