My wife recently bought a new phone and gave me her old one, it is an unlocked Samsung she bought a few years ago s/hand and has had no problem with. Put my SIM in and got the 'Network locked SIM card inserted' message. Contacted TalkTalk about 10 days ago who said they would send me a pin, nothing arrived so contacted again couple of days ago to be told they aren't doing anything as it is my fault and phone is locked. Had phone looked at by a local phone technician yesterday who confirmed the phone is not locked and TalkTalk should be able to sort it. Now nearly 2 weeks with no phone (email etc works fine) so not sure what to do now.
Your wife's Samsung 'phone - what mobile network was she using it with? The SIM card she or you removed should have network info or logo on it.
It's possible that the phone, although bought unlocked, has become locked to the mobile network she was using. If that network is Vodafone then TalkTalk will unlock the phone for you. Unlocking your phone from Vodafone If that network is not Vodafone then here's some advice Unlock your mobile
What network is your SIM working on? O2 or TalkTalk Mobile?
My SIM is on TalkTalk mobile, my wife is on Virgin but I was told on Friday that this phone is definitely not locked.
If the Samsung phone was used recently on Virgin Mobile then it will work on the EE mobile network.
The fact that you put in a different network SIM and get the 'Network locked SIM card inserted' does indicate that the phone has been locked to the Virgin mobile network.
Call Virgin on 0345 6000 789 and provide them with the phone's IMEI that you'll get by keying in *#06# on the phone call keypad or go to the phone Settings > Scroll down and tap About device > Status > IMEI information.