When my internet fails, it is no longer possible to make internal calls. 
How can I solve that (big) problem ?
I’m using plain Asterisk Ver.10.0.0
Configure your DNS\resolv.conf/etc/hosts correctly, so that reverse DNS queries, etc., either succeed or definitively fail.
Can you please give me a clue (link ?) how to do that.
Tnx.
On recent Linuxes, you also need /etc/nsswitch.conf.
The easiest to describe is that you fully populate /etc/hosts with all your local IP addresses, and set nsswitch.conf to use it, in preference to bind. man nsswitch.conf and man hosts. It may also be possible to improve things by setting suitable unavail actions in the former.
The better way is to run with a local caching nameserver and make yourself definitive for the in-addr.arpa sub-domain for your local network. The bind documentation is extensive and there also used to an O-Reilly book: DNS and Bind.
Change the files as follows:
/etc/nsswitch.conf
hosts: files dns
In the above makes sure files is first (it doesn’t matter if there are other items in the list to)
/etc/hosts
127.0.0.1 localhost localhost.localdomain
192.168.1.100 my.domain.name my
Add the internal IP address and fully qualified domain name of your server (and the short name - the first bit - on the same line).
hosts bind
is an old format. He should check the man page.
Yes. It is possible that it tries to resolve the address of the external server, as a forward lookup, which means putting it into /etc/hosts, or becoming a secondary server for the that domain.
I may be wrong about reverse resolving local addresses, but you should configure to allow this, anyway, as it is best practice.