Claiming to be a Telstra telling me that my internet is going slow!
Want me to go onto my computer so th can show me how to make the internet faster.

When I tried calling the number back, it did not go through!

This seems very phish to me

Don't answer this phishing fake number! If you do, you will probably get someone claiming to be from Telstra Internet Support /or Optus Internet Support /or Microsoft Internet Support.

They are generally looking for anyone vulnerable or gullible enough to log on to any internet server associated with their phone number.

HANG UP IMMEDIATELY..... then think "Did I report a fault?" And even if you had, if somehow it wasn't resolved at the time, would they call you out of the blue?