This Number is definitely NEXXA DEBT COLLECTIONS so don't answer.

How to handle the debt collectors in Australia [Disclaimer]
This is just my opinion and tips.
It should not be taken as advice of any kind and I take no responsibility for what you do with my information.
There is also a small possibility that my information may not be entirely correct. I don't condone people using this information to get out of paying genuine debts. I have no interest in straw-man theories, or using the argument that money comes out of thin air and the banks are all corrupt so therefore you should not have to honour your debts.
I also don't condone abusive behaviour towards anyone, regardless of how they treat you. Be nice and to use an aphorism "you will have clean hands." These tips apply to someone that has had their debt assigned to a debt collector -- that means no longer in the possession of the original creditor.
Tips predominantly apply to someone that is not satisfied the debt is their's and is not happy to simply set up a payment arrangement. Phone, snail mail and email correspondence:

Always be polite and respectful no matter how they treat you. Ask the staff member for their first name, staff number and email address.
Don't admit to anything until you are satisfied the matter is genuine.
If the matter is older than 6 years 'statute barred' and you acknowledge the debt or make a payment you can unknowingly reset the statute barred date and re-enter into a contract with them.
If it is statute barred they can not chase you unless you re-contract with them.
Ask them for proof. Don't just take their word for it. The burden of proof is on them, not you. According to my understanding you are entitled to a copy of the information they have on you, detailed statement of the debt in question, a copy of the contract they are insisting obligates you to pay them.
They must provide the documentation in a timely manner, and they can not take action against you whilst the debt is in dispute. When they produce the contract in question look for a debt assignment clause. If there is not one then you most likely have a way out there as well. From my experience I am yet to see a debt collector produce a copy of the contract they are trying to enforce.
If they can't produce the contract and other relevant paperwork I send them an email stating that they have been unable to substantiate their claim and to cease all contact until such time they can produce the documentation.
They might try and bluff you after this but stick to your guns. I always say my client admits nothing in the absence of proper documentation in relation to the alleged debt. 9 times out of 10 I never hear from them again.
If you can, always record the conversation for record keeping purposes but just make sure you tell them you are recording first.
Make sure after a phone conversation you follow up with an email to reiterate everything that was discussed and your understanding of it. Ask kindly for them to reply 'yes' if they agree that the email is a correct record of what was discussed. Side note: If your debt is over $5,000 and it is genuine, see if you can negotiate them down under $5,000 because apparently if it gets legal they can't pursue you under the bankruptcy act.
If you do admit to the debt upon supply of proper documentation, tell them what you can afford and don't let them dictate to you. If they say $100 p/w then there is a good chance (if you can't afford that) that you can offer them $20 p/w take it or leave it. Don't be tricked by their sneaky tactics. Such as them saying "acting on behalf of the creditor".
In a lot of cases they purchased the debt for cents in the dollar, the creditor wrote the debt off and the collector is actually acting on behalf of themselves. As previously stated. They most likely have never seen the contract they are trying to bind you to and are relying on you to accidentally contract with them verbally or in writing by admitting to the debt, therefore giving them legal right to pursue you. Admin nothing until they prove it.
If they say "Are you Mr Smith?" say yes. Then if they say I want to talk to you about the money you owe to ABC Creditor. Say in the absence of me sighting the contract etc. and satisfying myself that I owe money, you are alleging this is my debt and I admit to nothing and I have no business with your company, nor do I wish to contract with you.
They might then say "well did you have a credit card with ABC Creditor or not?" Say that is what you are alleging and I would like you to mail me a copy of the contract, full statements etc. to (wherever you live) so I can view them and respond. Please do not contact me until the information I have requested has been sent and I have had 14 days to view it and respond. If you can not produce it please do not contact me again.
Best of luck and remember it is ok to ask plenty of questions. Just because they send you an official letter with your account number and a familiar debt amount on it does not necessarily mean you have to pay them.
They could have stolen your mail and used your details to generate a fake official looking request - See more at: http://thearbitrator.co/post/fd0a6238-9588-4f05-829c-99d9754daa09#sthash.jZ6h5yoI.dpuf

FYI- the statute of limitations on collecting a debt is 6 years, so at the end of the day no one can make you pay if its over 6 years old, remember never admit to the debt or enter into any agreement to pay any collection agency, this would restart the statute of limitations and give you contract with them.

company is called nexxa - some form of debt chasers. he called my workplace today looking for an employee of ours. We don't give out any information though

I called back number 0466 711 079 rang once then goes to busy tone

Person by the name of Nick calls and leaves message to return call refuses to identify where calling from possible marketing or collections company

doesn't leave a message

Person by the name of Nick calls and leaves message to return call when I send message to identify which company. no response. possible marketing or collections company