This attribute contains digits which form the unique identifier of the
customer. It is the equivalent of the telephone number in the
telephone network (either native E.164 or X.121 address formats, or
Atm End System Address(AESA) in ISO NSAP format.).
For example, E.164 prefix 'e.613722', X.121 prefix 'x.302', NSAP
'45000441812460730F000000000020480D200100'.