If you can show the correct data on your layout, then both a calculation using & or a pair of merge fields on your layout should work.
In layout mode, select the text tool and use it to create a text block on your layout. With the cursor inside this text block, use Merge Field... from the Insert menu to add first the location code field and then the extension field. In browse mode, they should "run together" to form a single string of digits.
define an unstored calculation field with your expression:
RelatedTable::LocationField & ExtensionField
Use your names in place of mine and be careful to have the same Context (see drop down at top of specify calculation dialog box) that is exactly the same as the layout's context. (The name selected in "Show Records From" found in Layout Setup...)
BOOM! Thank you! I used the 2nd method and it is spiffy. I used FileMaker back in the day (pre-relational) and remember just enough to be dangerous. Now I just need to figure out how to create canned searches I can access via a button that search for multiple criteria automatically.... ;-)
Then you might find this thread of scripted search examples useful. It includes examples of multi-criteria searches where the records need to match all criteria (an "AND" search) as well as searches for all records that match at least one criterion (an "OR" search).: Scripted Find Examples
Thank you once again! You are a great beacon of light to those of us laboring in FileMaker darkness and ignorance... ;-)