Call fallback to the pstn, Architecture – Cisco ASA 5505 User Manual

ASA 5585-X, ASA 5555-X, ASA 5545-X, ASA 5525-X, ASA 5515-X, ASA 5512-X, ASA 5580, ASA 5550, ASA 5540, ASA 5520, ASA 5510, ASA 5500 Series