If the tables involved in the join are of 100M+ records what I do when the joins use varchar columns to improve the performance is to use an additional integer column of the varchar one that is a CRC of it (or hash if you prefer that) and use the integer one instead in the join.
If you use a varchar as FK then you're definitely doing something wrong from beginning. OP was talking about getting the phone number under certain conditions, and a phone number column is a varchar.
