When I call it back from a landline it says the service is not connected. Therefore suspect it is trawling for mobile numbers

Yeah dodgy as

Rang and hang up. keep getting missed calls from (03) 8204 xxxx numbers (Croydon Victoria) and they don't leave a message. I don't answer numbers I don't know and if it is important the caller will leave a message. As these numbers don't leave a message, and others like this have been confirmed as SPAM, I no longer answer them.

Every night same number same time I’m already the block this number

Keep getting calls coming from Croydon [city in Melbourne?] and can't work out who it is.

I keep getting calls from prefix 03 8204 (then random last four digits). When I tried to call back it said it was disconnected so I block numbers

Rang my mobile and then hung up.