You can try COALESCE(), like:

COALESCE({custphonefield},{mobilephone},{phone})

I'm not sure what your custom field name is, nor the mobile phone field name. But, in essence, put the phone numbers in order of precedence here ( the first one first, second one second, last one last, and the first non-NULL value will return. So, if {custphonefield} is not null, it'll return that value. If it is null, then it will check if {mobilephone} is null, if not, it'll display the mobile phone number. If mobile phone is empty, then it'll check {phone}, if all three are empty, it'll return null.