Jira Service Desk/Management does not allow you to enter a Customer Phone Number - or for that matter any other information about the customer - Name, Address, Phone Number, Company, Mobile Number etc.
These are all standard fields on other Service Desk / Service Management systems.
Its ridiculous to say you should have to manually create custom fields for these standard bits of information about your customers (or your users for that matter)
So the question is ..... Why ?