SIP: What is the best Local ringing policy for SIP UAs'?

 With this in mind, a UAC should develop its local policy regarding local ringing generation.  For example, a POTS ("Plain Old Telephone Service")-li SIP User Agent (UA) could implement the following
   local policy:

      1. Unless a 180 (Ringing) response is received, never generate
         local ringing.

      2. If a 180 (Ringing) has been received but there are no incoming
         media packets, generate local ringing.

      3. If a 180 (Ringing) has been received and there are incoming
         media packets, play them and do not generate local ringing.

         Note that a 180 (Ringing) response means that the callee is
         being alerted, and a UAS should send such a response if the
         callee is being alerted, regardless of the status of the early
         media session.

   At first sight, such a policy may look difficult to implement in decomposed UAs (i.e., media gateway controller and media gateway), but this policy is the same as the one described in Section 2, which must be implemented by any UA.  That is, any UA should play incoming media packets (and stop local ringing tone generation if it was being performed) in order to avoid media clipping, even if the 200 (OK) response has not arrived.  So, the tools to implement this early media policy are already available to any UA that uses SIP.

Note that, while it is not desirable to standardize a common local policy to be followed by every SIP UA, a particular subset of more or less homogeneous SIP UAs could use the same local policy by convention.  Examples of such subsets of SIP UAs may be "all the PSTN/SIP gateways" or "every 3GPP IMS (Third Generation Partnership Project Internet Multimedia System) terminal".  However, defining the  particular common policy that such groups of SIP devices may use is  outside the scope of this document.

References:
https://www.ietf.org/rfc/rfc3960.txt