Phone numbers are prone to spam hence they are made difficult to change.
Make sure phone numbers are registered in the name of the business. So prepaid numbers and phone numbers listed to a private individuals are seldomly accepted.
You can try local online databases of phone numbers. If the number can not be found online to belong to the business and the same address it is not likely to get approved.
Same problem I’m facing for addition of mobile no of my own business , mobile no is also registered on my name only… But still it is not getting approved by google team.
Why don’t google verify mobile no by sending a confirmation msg to the mobile no. itself.
If you are not officially acting as an agent for the business at their request I would recommend against doing what you’re doing. Don’t claim businesses that are not yours.