It depends. If your target audience doesn't require too much trust and they are willing to provide that information up front, then yes, ask for a phone number. To find out for sure, do one of the following two things:

*Get 100 (good sample size) sign-ups with email only and conduct a survey. In your survey, ask your audience if they would be willing to provide a phone number during their sign-ups. This way, you'll get it directly from the horse's mouth.

*Conduct a variant A/B testing. Ask for a phone number in your test group and see what the response rate is. The higher the response rate, the better the chances you can get the phone number.

So, start with the objective and work backwards by implementing one of the two aforementioned tactics.