datatype advice numeric vs. varchar

I'm working on a voicemail application which involves storing phone
numbers and other types that have long sequences of digits [0-9]. I
need to be able to do pattern matching using LIKE and map the fields
back to java String objects. I was thinking maybe it would be more
efficient to use the numeric(x,0) type instead of a varchar since it
could use less space on disk.

My calculations for disk space based off some information i found
online are ( 8 + ( 2 bytes for every four digits) ) for numeric and (
4 + number of chars ) for a utf8 varchar datatype. Are these
calculations still valid and has anyone tried using numeric for this
purpose or is this really stupid?

Thanks for any hints!
Gene