What about time zones, for customers calling from another continent?

The first text goes out the moment the call is missed, whatever the caller's local time: speed beats time-zone etiquette, and it's a reply to their action, not outreach. The single 4-to-6-hour follow-up respects your line's legal 8 AM–9 PM sending window instead. A customer calling at 3 AM their time gets their reply immediately, and the follow-up leaves in your morning. The sending-hours schedule is set per line.