Am I able to request that Optus report my account payments to a credit reporting agency (such as equifax) in order to improve my credit score (i.e. by showing that I make regular/on time payments) - customer service have been unable to answer this question.
Optus most likely doesn't provide what is known as comprehensive credit reporting (i.e. the good and the bad).
Mostly this process applies to the big four banks and other big credit institutions. Ultimately its the bad that has a big impact on your credit score. If you take out credit then if there are no negatives listed then its obvious you paid it out correctly.
Optus would only be involved in a credit reporting agency for when you paid a phone off over 1-3 years. They would generally only record a default (if that occurred). It has always been the case that you can request a record of payment to settle a default on your credit report.
Ultimately I wouldn't worry too much about it. As long as you're paying off various loans and not leaving unpaid utility bills etc. to pile up then your credit score will slowly move up.