E.164 is the international standard format for phone numbers: +[country code][subscriber number]

Rules: Max 15 digits total · No spaces, dashes, or parentheses · Always starts with +

In Asterisk/FreePBX: Use EXTEN=~^\+?1?[2-9]\d{9}$ patterns or normalize in the dialplan with Set(EXTEN=${FILTER(0-9,${EXTEN})})