A line-type flag records whether a number is mobile, landline or VoIP. It matters for connect rates — mobiles are answered more often — and it matters for compliance, because rules around automated dialing and messaging treat mobile numbers differently.
Line type is not stable. Number portability means a number can move between carriers and line types, so a flag captured a year ago may be wrong now, and re-validating is a running cost rather than a one-time enrichment.
The compliance dimension is the reason to care beyond connect rates: several restrictions attach specifically to calls and texts placed to mobile numbers, so a stale or missing flag is not merely a data-quality issue. This is a plain-English summary and not legal advice.
VoIP is the third category and the awkward one. A VoIP number may reach a person as reliably as a mobile or may be a routing layer in front of a switchboard, and providers disagree about how to classify them — which means a line-type field is only as useful as the provider that filled it in.
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