surepic:
storing 10 digits in 5bytes is possible
poc for 9 digits in 4bytes:
. . .
People used to program like that to save a couple of bytes when storing a date and caused a big mess leading up to 01.01.2000 
For storing a telephone number in a similar way, you’d have to convert the numbers to full international format (E164) and recreate the ‘+’ symbol, or add a flag indicating the number of leading zeros (if any).