Usually it will make matters worse. They then know that the number is a good working number which has someone replying (the unsubscribe/opt out) on the other end. I can't say that is the case for all spam numbers but I personally wouldn't take the chance. Your mileage may vary.
The Telephone Consumer Protection Act (TCPA), 47 U.S.C. § 227, restricts the use of the facsimile machine to deliver unsolicited advertisements. Specifically, the TCPA prohibits the use of “any telephone facsimile machine, computer, or other device to send an unsolicited advertisement to a telephone facsimile machine.” The TCPA applies only to those facsimile messages that constitute “unsolicited advertisements.” he statutory prohibition applies to such advertisements ent both to residential and business facsimile numbers.In 2005, the Junk Fax Prevention Act amended the TCPA to permit the sending of unsolicited facsimile advertisements to individuals and businesses with which the sender has an established business relationship (EBR) and to provide a process by which any sender must cease sending such advertisements upon the request of the recipient. On April 5, 2006, the Commission adopted rules to implement the Junk Fax Prevention Act. Among other things, 47 C.F.R. § 64.1200 requires the sender of fax advertisements to provide notice and contact information on the fax that allows recipients to opt out of future fax transmissions from the sender and requires senders to honor opt out requests within the shortest reasonable period of time, not to exceed 30 days.
The faxes I get with an opt-out number or fax are usually legitimate and are complying with the law. I have gotten a few with no opt-out information and have called the business to let them know they are in violation of Federal Law. This usually gets their attention. Neither of our fax numbers get that many unsolicited faxes, but I've never gotten another one from a company I've opted out from.