Cause No. 20 - subscriber absent
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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.