I have a client that keeps getting his phone number on his listing deleted by Google. It's the main business number and also appears prominently on his website. I see posts on the Google Forum but no solutions. Only thing I can think of is the firm does cleaning related to Covid but the listing is live....just no phone number.

Any ideas?
Anybody can change your info with edit suggestions. If it was Google then a lot of these people using generic GMB business names would be caught. Instead, Google does nothing, waits for it to be reported. 
Are there any apps that are allowed to update the listing?  They might be re-syncing the listing with their own data and eliminating the phone number.  Other possibilities include edit suggestions that Google might be accepting, or their NAP data is different on a major listing site.  

This issue seems to come up all the time, and I wish Google would explain the why behind data changes like this, so it would be easier to track down.