I found a cell phone when I turn it on I get a message saying "unregitered sim" what does that mean?

It means that the cell phone provider, cancelled the SIM which is the card that allows a phone to use a network. The SIM is the small card in the phone.

From there, you can determine the carrier. Has the carrier's name all over it. Remove the SIM and record the numbers. Call the carrier and give them the numbers and they should be able to find the rightfull owner.

Bear in mind that unless the phone is "unlocked", it will only work with the specific carrier ewgardless of whose SIM is inserted.

Sometimes you can scroll through the address book and find entries such as "mom", home" etc. that would allow you to indirectly contact the owner.

In my address book, I have entries called:

1 If Found call
2. Home
3. <friends name>

These show up as the first three entries, so it's easy. The 1,2 and 3 make them the first entries.

Dumb Cellphones:

Doesn't have First, Last and nickname