Sim card invalid - AT&T SIM Cards

It sounds like the SIM is corrupted. You may have to format it and start over.

What is phone brand and model?If the phone displays invalid sim card in screen it is locked to network,you need to unlock it.After unlock it you can use with any GSM network sim card in the phone.Without unlocking if you use other network sim card in the phone it will display invalid sim on screen.To get unlock code for GSM network locked phones visit the site Theunlockspot.com for iphones you can get remote unlocking service in the same site itself and unlock easily.

If the Nokia X6 16GB phone,displays invalid sim card message on the screen it is locked to some network.Please find out the network and country it is locked now.If it is locked to network,it will not accept other network sim card.But you saying it is accepting other networks sim card.How many network sim card have tried inserting in it?If you tried only one network sim card and it accepted that it is locked to that network.You can also try using some other network sim card.If it displays invalid sim message again.It is under network lock.To unlock it use unlock code.Visit the site Wickedunlock.com get code and unlock it easily.

If your Nokia X6 16 GB Displays invalid sim card while inserting other Network GSM sim card like MTN ?
It is locked to network.You need to specify which country and network it is locked now.I have bought an Nokia X6 16GB in INDIA but it was locked to Australia Vodafone network .I have unlocked it and now using it in USA AT&T network sim card.So you can unlock your Nokia X6 16GB from current network lock using the unlock code from the site Onlinegsmunlock.com after unlock it use with MTN network sim card or any GSM pay as you go sim card it will not display invalid sim card message.

If the Nokia N70 says INVALID SIM means it locked to Network ,you need to Unlock it...Get the unlock code at here Classicunlocking.com and unlock the NOKIA N70 easily then use any GSM Network Sim card...

LG invalid sim solution
model GS 190

If your phone show Invalid SIM it means the SIM card is corrupted you will have to change the SIM card.There is no other option.

i have a china made mobile n8 but after inserted it shows invalid sim what's the solution of this problem

Sciphone i68+ displays invalid sim

Invalid SIM and Unregistered SIM are two different problems. Invalid SIM means the phone is locked to a certain company and you are trying to use a SIM card for another company. Unregistered SIM means the SIM card you are using is not active and you need to contact your service provider to get it activated. In order to unlock the phone so you can use anyone's SIM, you must contact the provider that the phone is locked to.