The logic behind SIP expiration

Hi all,

could someone please shed some light on the logic of SIP registration. I experience that sometimes Asterisk reports all SIP phones as being “busy” and I suspect that this is because they are registering at that moment.

Since I have a failover configuration with two Asterisk nodes, offering a shared IP address, I’ve set the registration period of the phones fairly low, so this “busy” situation (if it is caused by re-registering) happens frequently.

But I don’t really get the logic behind the registration: I see it happening on the console, and all the phones seem to do this at the same time - is this because of the “defaultexpiry” parameter in sip.conf? In other words: is Asterisk responable for making the phones temporarely busy?

Thank you,