Can`t make outgoing calls in big load

Hello to everyone.

Here is the problem:

I have the Asterisk server, SIP adapters Linksys/SPA2102-3.3.6, analog ATS and analog phones.
There was an idea to make that half of people will make calls from analog phones and the other part will make calls from the computers using softphones.

So I registered SIP adapters on Asterisk server. When On Asterisk registered about 4 SIP adapters there is no problem, but when i`m registering 5-6 SIP adapters becomes a problem with outgoing calls.

There is a dead silence during the call. It`s can continue about minute and than call can be proceeded, or it can hang up, or the Asterisk can say “all circuit are busy now”.

I can`t understand the reason of that. Maybe someone already knows the reason?

In the log files of asterisk I found the record when the problem becomes: “SIP/2.0 500 Service Unavailable”

I hope i`ll find the answer here because I already have no idea what to do now :frowning: