Both sides tried to change the same call at the same time, a situation called glare. One re-INVITE has to wait for the other. The protocol resolves this on its own with a short random wait and a retry.
Paste a full SIP or Asterisk log into the decoder and ladder tools to see exactly where this response appears in the call and what led to it. Everything runs in your browser, so your logs never leave your device.
Open the SIP decoder See how a SIP call flows