I am trying to activate an iphone 5 that me service provider says is locked

How did you get the phone?

Often phones purchased off the street or through "clearinghouse" sites or Amazon have not be fully released from either the rightful owner AppleID or from a previous carrier contract.

Phones from friends may just need to be released from the device list of that friend and was just a missed step.

Here is a sale "checklist": https://support.apple.com/en-us/HT201351

Also, ask your co-worker to ensure by calling the previous carrier that they themselves unlocked it from THEIR device list. Carrier unlock policies get complicated, even for store-front employees, so this is often an unintentional mis-step.

How is it locked? What message do you see when you try to activate it?