How can I find the imei number if my phone has just been reseted and I don't have the box anymore?

I have reseted an iPhone thinking it was unlocked to be able to put an other SIM card.

Unluckily this one isn't unlocked so I can't put the SIM card I wanted and now I can't access to get the imei number in order to ublok it.

How can I get this number now?