Gender neutral alternative to Sir/Madam

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?" or "Would you please reboot your machine?" vs. "What can I do for you, sir?"
or "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.

Interestingly enough being in the military, especially saluting officers we say "Good morning, Sir" or "Good morning, Ma'am".

But at the same time I was thinking the other day about people of different genders or females that look more masculine and vice versa. What do I say then?

Simply put, just say "Good morning". Leave out the gender. Play it safe.

Merriam-Webster gives gender neutral title as "Mx" https://www.merriam-webster.com/words-at-play/mx-gender-neutral-title

So does Oxford. There is an mp3 there, demonstrating how to pronounce/use in speech.

There are few gender neutral honorifics in English that are going to sound completely natural in a conversational context, especially a call center.

If unwilling to work around the matter as others have suggested this may be a case where you could experiment with setting precedent for new cultural norms.

I have, on occasion, used the phrase my friend for this situation. However, that may be taken as too familiar depending on the circumstances. For example, I am quite put off when a politician with whom I have no other relationship addresses me as "my friend." Or even if this is done collectively as "my friends" when I am part of his audience. Yuck.

Hotels in their writing sometimes use the expression honored guest. Businesses more generally might adopt valued customer, esteemed patron and so on.

There's no grammatical reason these written forms of address can't be impressed into verbal service. And that they sound strange is largely a matter of becoming accustomed to them.