How to make IP phones not fall off?

Question

Asterisk

We have a server with Asterisk installed.
Periodically fall off for half a minute phones. Not all at once, but some. I did not reveal patterns, since they fall off in different places.
The error is this: The phone shows the entry "No registration". It takes 10-30 seconds and the phone is registered again.
No problems found on the network. In Asterisk is not strong, tell me where to look.
Phones: Yealink SIP T-20P
Asterisk 1.6.2.14

Answer 1

do you have many phones? try to pull them into a separate vlan