Slower operator holding up old phone company (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

USER(operator of a machine or equipment) placed below ( holding up , in a down clue) [ O(abbrev. for “old”) + BT(abbrev./popular name for British Telecom, a phone company). Defn: …/descriptive of one who is less intelligent/dimmer.