Found out that the zombie phonecalls are

[Ethan42]

computers calling many phonenumbers at once.

They are telemarketeers, dunno how you call them.

Once they have a connection on their screen, they are still connected to
the other people who picked up, but line seems dead.

I never checked though whether time of connection was on display of my
cellphone at that moment...maybe the computer instantly drops the other
connections, when another person answered.

Got two calls like this today. I saw on the display: 'number unknown'.
Didn't answer them.

Should get my phonenumbers in the Don't Call Me Registers again...

In Europe it's forbidden for companies to call anyone who signed up their
numbers in that Register.

Still some try...I guess.