It's worse than that. Not only are temporary bulk phone numbers cheap, phone verification is a profit center for spammers. Because they acquire a bunch of phone numbers for their own use, but each phone number can be used on any service that requires phone verification, so they can then sell phone verification service to other spammers or privacy-conscious individuals who don't want to give out their phone number, or set up a web page to do it that makes money from ads, and use the profits to expand their spamming operation. Not only that, the more services require phone number verification, the more profitable spamming becomes, because each number has an increased return since it can be used to sell an activation for an account on another service.

Meanwhile people who actually want privacy get screwed, because the spammer's account is going to get banned for spamming in less than a month either way, but a normal user would want to keep the same account indefinitely, and then the site demands that they keep access to the same phone number indefinitely. So then the honest users are stuck paying a monthly fee at the retail rate for a separate phone number for each service in order to avoid giving them all the same phone number to correlate with you. Whereas the spammers pay the wholesale rate once and then more than break even.

The anti-spam value of phone number verification is not just zero but actually negative. Its purpose is to harvest phone numbers from honest people for mass surveillance, and anyone requiring it is making the spam problem worse.