I am not sure about the process for your chosen provider... This forum is run by Telstra Retail (Telstra is split into two companies, retail who are the ISP and Telstra Wholesale who own the network and resell it to providers on the Telstra network) so I can give you general information but because each company has their own slightly different way of doing things it might not be completely the same with all providers...
If you are saying that there is a line that is working and active and can still be used for calls both in and out, then it means that the previous service hasn't been disconnected... which would mean you would need to follow the POD process which is that you provide proof of occupancy to your chosen provider who then get Telstra Wholesale to disconnect the old connection.
Once that is done then the line would be free to be used again for a new connection for yourself.
However, some providers may choose to have a second line put in or may have charge the $299 because they are putting you onto their own network in which case that is something you would need to discuss with them directly.
If you are saying that there is a line that is working and active and can still be used for calls both in and out, then it means that the previous service hasn't been disconnected... which would mean you would need to follow the POD process which is that you provide proof of occupancy to your chosen provider who then get Telstra Wholesale to disconnect the old connection.
Once that is done then the line would be free to be used again for a new connection for yourself.
However, some providers may choose to have a second line put in or may have charge the $299 because they are putting you onto their own network in which case that is something you would need to discuss with them directly.