Offhand, without taking into account the particular contexts of that recurring usage of “Are you understanding me?” by your Australian customers, we can’t categorically say that it is Australian slang or informal English. The only thing we can be sure of is that from a grammar standpoint, they were using the stative verb “understand” wrongly, making it take the progressive form when it really shouldn’t take that form at all.
As to their motivation for doing that, we can only speculate. Assuming that the Australian callers asked you and your call-center fellows that question in a pleasant, nonconfrontational way, then the expression “Are you understanding me?” must indeed be Australian slang or informal English to them specifically (but, mind you, not necessarily to Australians in general). Those callers may not be very English savvy and think nothing, or may not even be conscious, of their misuse of the stative verb “understand.”
However, if those Australian callers are well-educated and English savvy, there’s the unpleasant possibility that they didn’t seriously take a reply you had given them. Their question “Are you understanding me?” could, in fact, be of a different language register—a barbed and disparaging manner of expressing dissatisfaction or displeasure over your answer or perhaps over the faulty or hazy way you were expressing yourselves in English. You and your call-center fellows are therefore the best judge of the sense intended by your Australian callers in that misuse of “understand” as a stative verb.
To put the above comments of mine in context for other Forum members, let me just say that one not so widely taught attribute of the English language is that not all of its verbs can properly function in the progressive or continuous form (ending in “-ing”). This is why in the case of the transitive verb “love,” for instance, I’m sure that many of us felt a vague but strong discomfort the first time we heard the burger ad slogan “I’m loving it!” That’s not the right way to say it, my own inner ear told me that time; it could be the present-perfect “I’ve loved it” or the present-tense “I love it!” or the future-tense “I will love it” but never, never the wrong progressive “I’m loving it!”!
We cringe over that grammar usage because verbs like “love” denote a state or action that can neither be stopped nor started at will; they are typically involuntary actions that just happen automatically. Such verbs are called stative verbs as opposed to action or dynamic verbs like “talk,” “whisper,” and “grow,” which denote an unfolding action, process, or sensation. In contrast, stative verbs like “love,” “belong,” and “prefer” typically denote thoughts, emotions, relationships, senses, and states of being, and it’s unnatural or out of turn for them to take the progressive form.
So while we can say “Are you talking to me?”, “She is whispering to him,” and “Love must be growing between the two of you” with total confidence, we’d feel a nasty lump on the throat when we say “I am knowing the facts of this case,” “She is belonging to the sorority now,” and “We’re preferring fuchsia to magenta.”
This inability of stative verbs to take the progressive form is the telltale attribute that distinguishes them from action or dynamic verbs, and the better we internalize this, the better will be both our written and spoken English.
