If you don’t want to use an Address record to store a phone number (which would allow you to display a kind of directory), you can enter the number as an Attribute. Just add the custom type “Mobile Number”.
I don’t see a good reason for a custom attribute.
Simply add another address with only a phone number.
While 5.1 doesn’t show phone numbers (or postal code) in the Address tab, the new 5.2 was enhanced to do so.
One of the reasons to use an address record for this is that it supports a date range. So you can maintain a history of them. (So you might be able to determine who was being called for that number listed in the Family Newsletter contacts in 1980.)
See 0011600: Addresses tab can have invisible rows of Phone only records
Using an Attribute as @pgerlier suggests may be added to reports etc when you may not want this to happen. Making it Private would add some control. A person’s Attribute tends to get added to reports when an Address record is not.
Just because the Address can handle other information, you can limit your entries to just the phone number. Having it entered here you can aggregate all the Address entries in a report like the Narrative Web.