The built-in rules are easier to troubleshoot when each one has a single job. If routing, time-zone checks, and task scheduling are packed into one step, a missing field can stop the whole chain. Create a separate time-zone field and fill it as soon as the lead enters the CRM. Let one rule assign the regional rep, then have another schedule the callback from that field. Add a fallback task for records with missing location data so those leads don’t disappear without anyone noticing.