Requiring a cc for that is the wrong solution.
- If you don't give full access to what you're offering during the trial period, would-be users won't get a feel of what they might be losing out on when the trial runs out.
- If you do give full access during the trial period, but don't require some kind of hard to get unique enough identifier, a non-negligible users will create new accounts ad nauseam.
(Alternative unique identifiers could be SSNs or ID cards or physical mail addresses. But these are all non-starters. And phone numbers and emails are free for all practical intents.)
Can you speak more to your comment that phone numbers are essentially free? I know with something like Google voice you can get one free number (which still needs to be associated with a non-GV number). How would you get many phone numbers for free?
Also, the original comment specifically mentioned free phone numbers, not cheap or otherwise not-free numbers.
The negligible part is the cost of the freeloaders, there can also be value in freeloaders. They help spread the word and keep up the good word and they might convert rather than going to a competitor. They also represent a good opportunity to show good will.
The non-negligible part is the lost revenue from performing such shady practices as requiring a cc / phone number. If you require that for a trial I will perform extensive research of your competitors. And if desperate enough might consider you after that if nothing else was up to par.
If you allow me to sign up for a trial without an email that is very positive and shows me that you respect your users. That is very appealing.
And very reasonable, because the sole reason for anyone to ask for an email is so they can send spam to it afterwards. Which of course shows me that you do not respect your users.