They already charge $4 a month to block spam calls. Isn't this the same thing as charging for advertising? They are just charging the customer rather than the advertiser. The "feature" is called AT&T Call Protect Plus.
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AT&T Call Protect Plus
Want more protection? Upgrade to the Call Protect Plus 2 service for $3.99 per month to get advanced call-blocking capabilities.
When you upgraded, you'll get all the protection and features of Call Protect Basic and Plus:
Reverse number lookup: Identify caller details when you enter a U.S. number.
Caller ID: Get caller details, when available, in AT&T HD Voice-coverage areas.
Custom call controls: Choose what types of calls (like telemarketer, political, or nonprofit) to allow, block, or send to voicemail.