In spoken English for answering a call in a call center, you can avoid a gendered address simply by using repectful tone of voice and slightly formal phrasing:
"Thank you for your call. How may I help you today?"
"Would you please reboot your machine?"
"What can I do for you, sir?"
"What do you need, ma'am?"
In written English for a call center situation such as emailing a customer, you can use a greeting such as "Hello," with no title combined with formal phrasing in the body of the email.
Should you work in the rare call center where a bit of whimsy is allowed, you can use "gentlefolk" for a non-gendered plural, but this should be used with care.