My flip phone displays message "insert SIM" and

Your phone has to be unlocked to accept sim card from another carrier.

This is a normal behavior when a phone does not have a valid/working SIM card. Without a valid/working SIM card or even without a SIM card, the phone can still be used for emergency call.

I hope it helps.

You can insert sim card in LG sliding phone on sim slot.After inserting it displays any error message saying "Wrong sim" or "Incorrect sim" or "Emergency calls only", etc., then your phone is probably network locked.To unlock it,visit the site WickedUnlock.com here they are providing the unlock code at reasonable cost with easy step by step unlocking instructions and unlock it.This is the fast and permanent unlocking method.
Source:http://www.wickedunlock.com/lg/rs16wp8/

If your Samsung phone displays an error message saying "Emergency calls only" when you insert a Sim card then your phone is probably locked. To unlock it,visit the site ClassicUnlocking.com here they are providing the unlock code at reasonable cost with easy step by step unlocking instructions and unlock it.

Its the problem with your sim.May be the phone is unable to read the sim. Insert a new sim to check. If problem persists go for a hardware check up at your LG service center.

In my area that usually means that your phone service has been cut off.They provide only emergency calls as a free service.Sounds like you need to get in contact with your provider and find out why your phone seems to be cut off.

Hi friend, how about enter the setting. >Then restore factory setting.> Going 4G or 3G Strong signal place. Then try again your SIM card to your Samsung. May be it can useful to you. For more Samsung useful tips here : Tray Installation Samsung Galaxy S8 Plus

ive got a mobicel d2710 and when i forgot my unlock code for the settings menu.pls help

After inserting my SIM card, switching on the phone, "Insert SIM" is displayed and when trying to make a call, the message "Emergency call only" is displayed and

Sounds like the provider (AT&T, Verizon, etc) has that particular SIM card (and thus account) suspended or otherwise disabled. Call them up or stop in and find out why that would be.