2011/4/17 Juha Heinanen jh@tutpro.com:
lets say that a sip ua has dialog established with pstn gw and the sip ua sends refer to pstn gateway for the purpose of transferring the call to another pstn destination. in that case, referred-by uri is used for accounting of the new pstn leg.
I've never seen a PSTN gw properly handling a REFER, neither I think a PSTN gw should handle it (but a B2BUA/PBX) between the UA and the PSTN gw(s).
The REFER with "Referred-by" headers tells the receipt of such REFER to generate an INVITE with destination indicated in "Refer-To" header and mirroring the "Referred-By" header (so the UAS receiving the INVITE knows that it arrives from user in From header but is a transfer from user in "Referred-By").
Honestly I can imagine how a PSTN gw is supposed to assert the value of "Referred-By". Perhaps it just allows the REFER if it contains a Referred-By matching the From URI header?
Cheers.