Only goes to voicemail and then hangs up

They called offering to build a website for my domain that i purchased. I put my number (real number) on the whois detail not knowing the repercussions. An Indian lady was on the other line so this seems like a dodgy spamming number.

I have received several calls from this number. Each time there is a ping sound and then an automated voice saying "good bye."

I would like to know why this sort of system exists. Where is the incentive to call you, be charged for that call and collect only the data that it is a real number that gets answered.

Is it sampling my voiceprint perhaps? Harvesting my voiceprint as a biometric to be used to hack an account of some kind? Waiting for me to answer and say my name so that when a telemarketer calls they can pretend to know me?

Nothing related specifically to me and nothing logical as far as I can tell.

What do they achieve by calling and hanging up??
It’s a mobile they ring not a landline they just rung me on.

I tried calling them back on a private number and was told that "the number you have called is not in service - please check the number and try again".
Weird because there was only 10 seconds between them calling me and me trying to call them back.

I called back number (02) 1136 4983 rang 2 times then goes to busy tone.

seems doggy, never leave a message