20sec delay when calling and lots of account re-register

Hi everyone,

Recently I have a problem bother me long time, I use pjsua to send a sip call to an ip phone, and then after almost 20 sec, my ip phone start ringing. When this problem occur, I checked the asterisk log, I saw many of sip accounts re-registered.
This problem occur very randomly, last time is 4 months ago, and last 2 time is 6 months ago, I restart asterisk then it work normally, but yesterday this problem occur 10 times in one day, I restart asterisk but few minutes later, the problem occur again.
My system has almost 1700 sip accounts, run in a local network.
My server is Cisco C220 M4, asterisk version is 14.3.0.
Could anyone give me some suggestion?

Following pictures are screenshot of asterisk log and extension.conf and sip.conf
Lots of accounts re-registered

receive sip invite then 20sec later start dialing

dial plan is very simple

sip.conf setting

This will happen if there are DNS problems. The chan_sip module blocks while doing DNS Lookups, and thus can block other traffic and actions. That would be where you should focus on investigating.

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Thanks for your reply.
I known DNS issue may cause this problem, but I have no idea about all accounts re-registered, could you give me some direction to investigate this problem, thank you.

When chan_sip is blocked on DNS, any inbound SIP registrations will be blocked. Depending on how often registrations occur, when, and how long chan_sip was blocked then that can occur.

Got it.
We didn’t use any domain name setting except the mailbox, but we didn’t set voicemail in dialplan, it still do DNS lookup?
Now we delete the DNS server in network setting, and monitor our service continuously.

It will reverse lookup IP addresses.