Yes, it's a scam. And it doesn't matter if they say it's not a sales call, it is a sales call and/or it's a scam. And many sales calls for septic tank products are scams anyway. You give them money and they send you some dirt in an envelope....you can get the same thing outside for free.
Never call the number back. Scammers will never remove your number or put you on a do not call list.
If they are outside the USA you will be hit with high charges and could be put on hold to run those charges up even more. Some scammers also collect active phone numbers this way and sell them to other scammers.
Best to ignore the calls or block them in some way.
Got a call from this number. I am on the do not call registry. Is there a way to stop these calls. I don't answer because I don't recognize the number.