"No Service" I'm replacing my Motorola i850 with a new i880. When I place the used SIM card, everything work but it shows "No Service" on the screen. I'm just upgrading the phone. I have a contract with Sprint/Nextel. Any help?

go messege settings then text messege

If you are the first owner of this phone, then 0000 or the last four digits of you phones number should work. But try the above even if your not the original owner.

When you place a new or a existing SIM card from another phone provider into your mobile phone and it asks for a Security code then :-
This phone is locked to the original mobile phone provider. You need to contact this mobile provider to ask for the unlock code (security code) for this phone. You need to provide proof of ownership etc and the phone's unique IMEI number which is located inside the phone under the battery. They use this number to generate the unlock code for your phone.
They should provide you with instructions on how to enter this code into the phone.
Once you unlock the phone, then you can insert a new SIM card in this phone or sign up with another phone provider using this phone.
You could ring the phone provider and ask for the unlock security code because you are going overseas and wish to use your phone with a prepaid SIM card from another country. This maybe a free service.
The other way to get your phone unlock is to use a phone unlocking company on the Internet, they usually charge a couple of Dollars for their services. Use Google and search for "Security unlocking code” companies in your area.
You can also try this WEB site :-
http://www.mobileunlockguide.com/mobile-phone/

GOSTARIA DE SABER COMO TIRAR CODICO ESPECIAL DE NEXTEL

I have a Motorola i880. When placing sim card in the phone it is requesting a special code

Sorry there are no subsidy codes for the i850. This means that you have used the wrong type of SIM Card. You can only use a Sprint Nextel or Boost SIM Card in an i850. If you are using a correct SIM card, you will need to have this repaired at a local service center. This is covered completely by Sprint Nextel's ESRP (Equipment Service & Repair Program) and TEP (Total Equipment Protection), and can be done at any Sprint Nextel Service & Repair Location. If you do not subscribe to ESRP or TEP, the charge for the repair will be no more than $55.

You have to have special software and the cable to do this....

You r new pone sounds like its damaged take it to a service center

unregistered sim card? if thats the prob. maybe som1 else is using ur phone # like a replacement sim. if not check the network frequency