Assume you’re talking to the right person, don’t ask. If it’s the wrong person, they’ll tell you. (e.g. “Hey James, this is Joe Weathers with Relate Realty.” Instead of “Hey, is this James?”.
If you mess up a voicemail, hit # and most of the time it will give you a menu option to re-record the voicemail.
Don’t say “I’m not the listing agent.” That has a negative indication, as if you’re letting them know something that’s disappointing or less than ideal. If they ask “Are you the listing agent”, say “No, I don’t represent the seller. Realtor.com connects you with a buyer’s agent in the area since the listing agent is representing the seller’s best interests.”
If they don’t ask, proactively bring it up yourself before the end of the call to let them know you can help with any homes they’re interested in. Something along the lines of “BTW, I don’t represent the seller, so I can help with looking at any other homes you’re also interested in seeing!”