We've been getting a number of 404 Not Found errors on our SIP trunks, they tend to come in clusters at certain times. Here is one example: Call or Registration to 07xxxxxxxxx@(Ln.10000@TalkTalk SIP) has failed. [ V4 xx.xx.xx.xx:5060 UDP target domain=unspecified mFlowKey=904 ] replied: 404 Not Found; from IP:xx.xx.xx.xx:5060 (I've blanked out some bits for security with x's) Trying to decipher the Activity log, is the 404 error because... 1. The 3CX could not access the TalkTalk SIP or 2. TalkTalk could not access the 07 number. I have the detailed Activity log for the above event but would rather not post it on an open forum and its a lot of x's to insert!