Table relationships using phone number

I want to build a relationship between two tables using a phone number field, but a phone number field can't be used as a key field.  If I convert the phone number to a text or numeric field using a formula, formula fields can't be used as a key field.  Is there a word-around to this so that I can set up a relationship for the tables using the phone number as the linkage?

The best I've been able to come up with is a report link field that displays a link from Table 2 to Table 1.

Table 1 contains customer accounts with a Key Field is Customer ID.  This key field can't change because record updates are imported from another system.

Table 2 contains phone call records with the standard Record ID# as the key field.