Can the urgency threshold differ across my lines or hours?

Yes: each line has its own threshold, adjustable from 1 to 5, changeable as often as you like — it's a slider, not a contract. You can keep 4 during the day on the "emergency" line and push it to 5 on Sundays so only floods get through, or drop it to 3 on a second site's "quotes" line. The change takes effect immediately and the threshold history sits in the audit log. That setting is yours — nobody else decides what's worth interrupting you.