during takling between extention if internet line is disconected for a short tine the both extentions (lines) remains busy and it’s impossible to call again while asterick is not reloaded.
This is due to the way Asterisk processes SIP, basically it’s a single thread, so if it runs into an issue with being able to resolve the DNS of a remote provider that thread locks up and ends up blocking call processing. There are a couple of fixes, you can use IP addresses for your SIP Trunk providers, removing the need for DNS lookups, or you could use the method the FreePBX Distro has in place, which is to install DNSMASQ which makes sure your system maintains DNS in the event of an internet outage, or DNS outage by your provider.
Asterisk 12 will be implementing a multithreaded SIP stack that should correct this bug.