Over the years there have been a lot of postings about this problem. It looks like there may be multiple causes. Looking at some sip debug traces it seems clear to me that it is sometimes caused by a flakey pstn line and a sensitive SPA3102 trying to decide if the call was answered by a device on the line external to the SPA3102. In other words a SPA3102 monitoring the line voltage and current then deciding that the call was answered external to the SPA and discarding any caller id associated with the call. Then the SPA3102 senses more ringing and starts ringing a new call without any caller id. At least that is my theory. I am not aware of the solution.