Welcome to the community!
I'd like to confirm if the handset is directly connected to the modem/router phone port without using Filter/splitter. Also have you tried turning on and off the modem/router?
'By the way, am I required to contact Telstra to cancel my previous phone line since my number has now been ported over? Or will it be cancelled automatically?'
Yes, you need to contact Telstra to process the cancellation of your previous phone line.
I tried to use your community details to pull up the account and I got multiple match.
Can you PM me your account details (Username/Customer ID or Contact number associated with the account)?
In case you need a reference. How to send Private Message.