The request was accepted for processing, though the outcome is not final yet. Most often seen when a REFER for a call transfer or a SUBSCRIBE is accepted. Note that newer specifications discourage its use for REFER, but you will still see it in the wild.
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