+91, ten digits, first digit 6 to 9. Most of the mess in a CRM comes from the same four mistakes.
India's country code is +91. Mobile numbers are ten digits and begin with 6, 7, 8 or 9. Written in full international form that is +91 followed by the ten digits, with no leading zero.
Landlines are an STD code plus a subscriber number, together also totalling ten digits — so +91 11 followed by eight digits for Delhi.
Every one of these is common in imported data and every one of them breaks dialling, deduplication or both.
Keep every number in full international form with no punctuation — plus, country code, digits. That is the form a dialler wants and the form that deduplicates reliably.
Format it prettily on screen if you like. Just never store the pretty version, because the pretty version is where the duplicates come from.
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