I lost my iPhone today, can somebody else use it?

I lost my iPhone today so I called ATT and they turned my lost phone off and will be sending me a new one. For the person that finds my phone will they be able to use it if the put another sim card in?

Sorry for your loss.
ATT has a clever feature (Some would say devious) that restricts your old Iphone to their ATT network.
So, if someone puts in an ATT SIM card and tries to make a call, ATT will recognize your old phone's Electronic Serial Number (ESN) as lost or stolen -- and the ATT network won't allow the lost phone to operate.
If a person puts a TMobile or prepaid GSM carrier SIM into your lost phone, the phone won't recognize the SIM as an ATT SIM. And it won't work either.
Some sophisticated hackers may be able to disarm these locks but the general public can't.
I wish you good luck with your new phone. Just for our information --- What did it cost to replace with a new phone from ATT?

No. The phone is locked to ATTs network. And the bonus is guess what? When you called and had it shut off that phone's IMEI or serial number is now blacklisted meaning it's in the US database of lost/stolen phones and will never be reactivated in this country ever again. And if you have an iCloud account loaded on it, they won't even be able to try an activate it with another sim card because they don't have your iCloud password and won't call you. So it's a paperweight now. Best luck.

Depends on what SIM is put in. Anything other than an AT&T SIM will not work. If another AT&T SIM is put in, then they would have to get past Activation Lock, which is basically impossible.