I had an IH last night as the VC yells "sub" as official was handing ball to player to inbound after a timeout, during boys' C (Yes I chastized myself for the IH & tried letting the officials know it was an IH, however they let the sub in)
If they stopped the game on your horn, then they didn't have any choice but to let the sub come in. Because it's a dead ball it now become a legal substitution opportunity.