Cause No. 20 - subscriber absent

From: https://networking.ringofsaturn.com/Routers/isdncausecodes.php

This cause value is used when a mobile station has logged off. Radio contact is not obtained with a mobile station or if a personal telecommunication user is temporarily not addressable at any user-network interface.

What to do ?

• Restart phone which suppose to receive that call.
• Enabled Qualify if Device looses connection constantly.
• If Provider returns this error - try to change Provider

See also

• Hangupcause Codes