The call could be a scam or could also be legitimate.
Most contracts entered into allow information to be shared with other parties needed to fulfil the contract. For example, if you order goods online, the online company provides your contact details to the courier service to allow the item to be delivered.
Likewise with Optus. It is plausible that they shared your details as allowed under the contract terms and conditions, with another party to allow the contracted services to be delivered.
This many include subcontractors who make the connection to your residence on behalf of Optus. Subcontractors may also contact you directly in relation to any work that needs to be performed at your residence.
Now, the above assumes that the call is legitimate.
It is also possible that the call not legitimate and is a scam. Scammers will be able to work out where nbn connections are likely to be occurring and target these areas. If you beleive that it is a scam, I would be asking for their name, company they represent and call back details so that you can call them back. Then say that you will call them back when it is convenient to them. Having their details (if provided), will allow you to check with Optus their legitimacy and whether the call was a scam. If it was legitimate, you can then call them back and progress the works. It is likely a scammer won't provide such details and may hang up.
I would never provide any personal details to them than they already have, such as banking, credit card, date of birth etc. Such automatically would raise flags as being a scam.
Also it is important to check the terms and conditions for any contract one enters, including for goods, services or even competitions. Many of these have conditions which allow information to be provided (or sold in case of competitions) to third parties. In some cases, passing of information on is needed to facilitate the efficient progress of the contract...in some cases such as competitions, it can result in 'unsolicited' contact and spam.
There are also questions you can ask them to verify they work for Optus, such as date contract was made, contract, accoint or order number, details of the service/goods to be supplied and the contract contact person in Optus.