I would recommend connecting to Wi-Fi and make sure that your phone is updated to the latest version of Android OS available. Also check to make sure that your carrier settings are up to date as well. This can sometimes be caused by the Android software trying to use the Phone app and not having something that it needs to continue, so the app fails and shuts down. Hope this helps!
A couple of things to check would be to make sure your SIM card is active. If it is, then disregard that. Check to make sure your SIM reader is working... If you remove the SIM card it should switch and say NO SIM. If this happens, leave the SIM card out for at least 30 seconds and then put it back in. If it doesn't work after doing this... You may have another issue. I have seen where phones that are slightly bent causes the phone to not be able to find service. I do not recommend bending your phone back into position with the screen still on, you will risk breaking the screen. If none of this applies to your situation, check your IMEI against the national database and make sure your IMEI is not "blocked" which makes it so it cannot be used on any carrier's service, even it's original. If this is the case, and you're the original owner of the phone, a quick call to your carrier's customer support line should solve this issue.
As always, disconnect battery/power. The black glossy plastic area that surrounds the keyboard is held in place by snap tabs. Pry along the side until it pops off. I believe you have to come in from the back to disconnect it from the motherboard from the underside, which is barely guarded by 7 philips head screws, and 1 or 2 more underneath the battery. The case is also held in place by the same type of snap tabs as the keyboard on top. From there you can disconnect the keyboard and lift it out from the top. There might be a cd drive and/or harddrive that needs to be removed before. I can't remember, since I'm typing this on one of these, I can't exactly do this and type this answer.