You may not need any field at all here. If your fields are stored in a related table in the same record as your matching value in field A, all you need to do is place the fields that contain your phone number from the related table in your layout. This assumes you have the following relationship:
MainTable::FieldA = RelatedTable::FieldA
Thanks, Phil. I was able to figure it out myself in the meantime. I set up a table with the values I needed, created a relationship between that table and the one I was working on and then set up those fields as Look-ups. It worked great!
Lookups work but are very likely to be redundant. They may even result in the wrong info being displayed.
Key question, if you change a value in the related table, do you want all records in your main table that link to that record to automatically show the changed value? If so, then a lookup is NOT what you should use in this case.