Asterisk continue using Zap phone line after hang up

I have an odd issue with TDM400 card. Pretty often after hanging up the SIP call to external phone line the Asterisk keeps that line busy so no standard phone line incoming/outgoing calls can be accepted/made.
Is there any way to troubleshoot/debug/resolve that issue?

Thank you in advance

make sure your line has disconnect supervision enabled. the way to test this- get a normal analog phone with a light up keypad (but doesnt use batteries or power plug). Call your cell, and hang up the cell. When you hang up the light on the keypad should go out for a sec.

If this does not happen, call your telco and request disconnect supervision be added to your line.

If it does, make sure the line is set for ‘kewlstart’ signalling, and disconnect supervision is enabled in zapata.conf.