My blackberry bold 9000 can not hold network for a while, and it can not mke call or receive call.

To Unlock Blackberry Bold 9000 use unlock code..Visit the site Simpleunlocking.com get unlock code and get free Unlocking instructions at Mobileunlockguide.com/blackberry-bold-9000 and unlock it from network lock step by step.

Your Phone locked to Network,So it not detecting other Network Sim card in the Phone,You need to unlock it,You can get the Unlock code from here
Theunlockarena.com and Unlock it,then you can use it any GSM Network Sim card,Specify the Current Network lock,Country and Get the code and Unlock it...

You can get the Unlock code at here http://www.theunlockarena.com/blackberry-bold-9000/rs13wp9/
and Unlock the Mobile...

you can check it with other sim card ,if it remains the same problem ,it may be locked to particular network wont accept any sim card other than the particular network ,you can unlock it and used to other network,try here http://www.prounlocking.com/blackberry-9000-bold/rs8wp9/

Call AT&T customer service - they will give you the unlock code for free

GreenCells

Enter http://blackberrymep.com and follow instructions there, you will get your blackberry unlocked in 5 minutes .

There can be many ways to Unlock your phone.As far as i know unlocking by codes is the simple and safe way.You will need the unique MEP Code for your Blackberry BOLD 9000 and to get it you can use a website with service for Blackberry unlocking .so you can use a website to Unlock your Blackberry BOLD 9000 online instantly - OR local mobile shop .they are mostly using cables to unlock it and the price is higher then on unlocking websites but the result is the same: SIM free Blackberry.

Hello
You can call the network provider for free unlock code.
Or buy the code from third parties.