I think you have this backwards. E-mail is actually a good example of why you shouldn't open up communications systems too much. Because they're more or less free to send, people who want to send advertisements send loads of them. This makes e-mail entirely unusable unless you are very proactive in restricting who can send mail to you. Technical limitations like SPF, DKIM, and so on only prevent the worst abuses. What does really work has been IP blacklisting and reputation systems that more or less make rolling your e-mail very difficult.

Yes, you can roll your own e-mail; but you're taking on the challenge of both getting spam out of your incoming mail as well as getting your outgoing mail to be deliverable to everyone else. As a homelab[0] training exercise, it's fun; but businesses that need reliable mail just outsource it all to Google or Microsoft. The end result is that e-mail users more or less reinvented the restrictive systems that phone service used to have before the FCC opened POTS up to everyone that wanted to call an entire state about the their car's extended warranty.

When you mention spectrum limitations, that's for providing mobile phone service; which is only tangentially related to the actual phone call routing these days. Just getting a dialable number or placing a call is hilariously cheap and plenty of services of varying quality will let you do this in bulk. Providing access to that number over wireless spectrum is the expensive part; but you don't need spectrum to spam people.

[0] Don't try to take the word "homelab" literally and run your mail server on your residential ISP. It won't work.