said by Mr Herb :
Go to Walmart or your favorite boating supply store and buy a canned air horn.
I've read that the way to circumvent these noise filters that they all use to prevent that from happening is to answer the phone as you normally would, let them get to the point where you show an interest but start having problems hearing them.
"I'm sorry, I didn't hear that, what did you say, I cannot hear you clearly, would you please speak louder"
The majority of these operators will turn off their filter protection believing it's safe to do so.
You'll know when that happens, the voice levels will markedly increase.
Enter the fog horn.
I wouldn't recommend anyone doing this, it can cause injury to the other party.