ok, using the OP as an example:
she may prefer Jacinta, Jac, Cinta, Cin (these are all names i have been asked to call Jacinta's through work or in my own life). She may also prefer Miss (surname) or may use her middle name as her preferred name.

i know it sounds redundant sometimes, but it's part of building rapport and showing respect to customers. you can't assume because someone says their name is XYZ that they will actually happily accept you calling them XYZ. unfortunately there is a significant proportion of our population that will call themselves XYZ but expect those in service industries to address them as Mr/Mrs/Miss/Dr ABC. and quite a number of them will put in formal complaints against the consultant for being "too informal" in their interactions - so it's often a question that needs to be asked. I don't do it in my current role, as i take whatever name people give me as their preferred name - but when i was working with ZF, we either HAD to call them Mr/Mrs/Dr/Miss OR ask if it was ok to use their first name - and it was expected that we use name at least twice (or three times - can't recall) in every call (if we didn't, it wasn't deemed to meet quality standards and if that call was monitored, no matter how "good" the call was, that aspect had you losing major points off your quality assessment - so it became second nature, no matter how someone introduced themselves, that you'd double check...
This.
It's not just Telstra that do it too, it's a lot of call centres - I worked at Telstra for a while before moving to an electricity retailer and we also had these type of standards. Mind you, we were also permitted to use "mirroring" so if someone said their name was Jacqui instead Jacqueline or directly told us that they preferred it, it was considered permission to address them that way provided no surname was given. If a surname was given it was expected that we ask how they preferred to be addressed. Same goes for names that are very difficult to pronounce, we would ask for clarification on how to pronounce the name if it was spelt out, and were permitted to use any preferred nicknames or shortened versions.