Stolen phone

See the following If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

including the excerpt below:

Find My [device] is the only way that you can track or locate a lost or missing device. If Find My [device] isn't enabled on your device before it goes missing, there's no other Apple service that can find, track, or flag your device for you.

Depending on the region you may try talking to the network operators as they are probably the only ones, who can see the IMEI on the network directly.

Some network operators have IMEI blacklists, you can get it blocked, so at least the thief doesn't get any benefit from it.