I have never used a stamp but I have had a few situations in which an option was not given. Examples I have had are Marriott UK (they have used non-geographic telephone numbers too, a gigantic ripoff), Harrods, HSBC China ATM, St Regis Lhasa etc.

In every case I have had easy reversals. HSBC was instant (I am HSBC Premier, probably helped), Chase and Citi were both nearly instant and I did not need to provide any documentation for them.

I have no proof, but I suspect that for premium cardholders who also travel a great deal most major issuers have an almost automatic reversal of DCC.

For hotel users we might want a discussion of non-geographic numbers also. I am not an expert on this subject although I was victimised by it. Marriott, despite my then Platinum status, refused to reverse the charges for me which were roughly GBP 1000 over a three week stay. I then disputed the charge and the dispute (on a Citi issued card) worked. Do any of us know this scam well enough to offer definitive guidance?