In Baymard’s usability testing:

Required but unexplained phone fields were observed to be a direct cause of abandonments for the subgroup of users who are suspicious of a site’s motives.

 
58% of brands don’t provide an explanation.
 
Attached below is a poll done by Baymard to 2800 shoppers.
 
 
So…
  • Reconsider whether you actually need their phone number
  • If you do, then give an explanation as to why the phone number is required. (e.g. “used in case of delivery issues”, or “For order questions only”)
 
Baymard suggests 3 options to alleviate user concerns about giving away their phone number:
  1. Short inline description
  2. Tooltip
  3. Optional phone number

 

(Source)