Our loan department is looking to give customers another avenue to pay their rate lock/float fee instead of just paying by check. They would like to be able to do a one-time ACH debit from the customer’s account (from another bank) and be able to authorize it over the phone. Is this allowed?

You would be creating an ACH entry that falls into the 'TEL' SEC code. Single-entry TELs require the originator to create "an audio recording of the oral authorization, or provide the Receiver with written notice confirming the oral authorization prior to the Settlement Date of the entry." I would highly recommend checking out the 2013 NACHA rule book section 2.5.15.