i bought an iphone on second hand. There was no apple ID signed . And i tried to delete all setting, contents and it required restriction code so that i couldn't factory reset my phone. Anyway, i signed my apple ID then i decided to reset it with com

iOS has deep embedded software recognition built in that cannot be changed by simply by resetting specifically to discourage the practise of simps factory resetting of stollen phones.

The iCloud sign in is remembered outside of the base iOS.

The only way to remove this is via the original iTunes account with which it was paired.

When buying a used iPhone you MUST ASK to seller to confirm that they HAVE removed all their data and un-connected it from any previous iCloud accounts .And that it is unlocked - BEFORE exchanging with money.

only the previous owner can unactivate it

could be he sold you a stolen phone though and that is why he don't answer the phone

If the phone was stolen and quickly sold on - sorry you have been scammed and ripped off.

This is exactly how the security function is supposed to work - The phone to you is useless !

Because Activation Lock was turned on with that iPhone. Once you tried to restart it to clear the restriction code it went to Activation Lock.

They won't help you beyond confirming if it is a stolen device.