Have them look at off hook (when they are dialing) versus someone who is actively engaged in a call where answer supervision has been provided to the party.
So - I call mom, but mom is just dialing someone or she dialed the local pharmacy and the pharmacy has not yet answered....those calls cannot receive a call waiting tone.
I call mom and she's talking to the pharmacy. She should get a call waiting tone and I should hear ringback.
So off hook is different from "engaged in a call" and the SS7 links may be passing different header information about the call that is being mis-interpreted by the "Voipo switch".....
Just my 2 cents.
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