[Serusers] SIP call hold/unhold

Dear All, I encountered a UA that behave as follow: UA1 call UA2 UA1 ---- invite with SDP -----> UA2 UA1 <------OK with SDP --------UA2 UA1 hold the call, UA1 ------invite (hold) ------>UA2 UA1 <--------OK but not SDP ---UA2 UA1 unhold the call UA1 ------invite ----------->UA2 UA1 <-------OK but also no SDP---UA2 I found that most ATA(UA) would attached a SDP when reply an OK whether the call is hold or unhold. But there is one ATA that does not (UA2). I noticed that some ATA--Linksys (UA1) would send a BYE to UA2 if it receive a OK with no SDP in the hold case. My Question is : Is UA2 behave according to RFC? Would deeply appreciate if someone could let me know the answer as I always have trouble reading RFCs... Thanks fot any help available. Regards, TC Chan