I have not conducted a recent study of Tier 2 data vs. other contact data sources, so cannot speak to its quality.
Two other vendors directly collect and verify contact direct phones and emails. If direct-dial accuracy is a key concern, also evaluate DealSignal and SalesIntel.io. DealSignal performs overnight reverification so is better for marketing than sales. SalesIntel performs 90-day reverification cycles and claims to be significantly less expensive than Zoominfo. Both companies offer contact enrichment, contact prospecting, and enterprise software connectors (CRM, MAP, Sales Engagement, Chrome).