Pick it up. .twice in 2 days..after 3 seconds of silence  call drops. .scam or robocall to see if somebody will pick up. .

Same thing happened.

2nd time. I don't answer and no message is left.

It's a fishing trip, trying to get you to press '1' and it shows a real person and now you will be slamed with a live caller.   Do NOT press any number, put the unwanted caller on your reject list, create a contact named 'SPAM' and hang up.
NOTE: Robot spam calls are on a auto-dialer 24X7.  All cellphone and land line phones your carrier sells your information. YOU have to write a written letter and mail to your carrier to opt-out of all afflities sharing your info.  Most of your calls will stop.  Even if you put your numbers through the donotcall.gov.

Just called at 11:27 am. No message left. Calls often.