Iphone problem

1st, check to see which phone you have. If you have 3G, then the firmware on it is 4.2.1. You can't have a 5.0.1 on 3G.

2nd, you can use redsn0w to hactivate it so it can by pass the activation screen.

 

Sounds like he updated a 3GS with the ipad bb to stock 5.0.1 firmware.
Sure, update a JB and unlocked iphone and it will still work with Tmobile

 

Very sure.
The highest firmware for an iphone 3G is 4.2.1

 

yeah, 100%. Like AJ said, your phone has an iPad baseband on it and you upgrade, restore it to stock 5.0.1 and therefore, your receive a no sim message. Even you have original sim, you can't even activate through WiFi.

 

Run redsn0w 0.9.10b4 and it should hactivate your phone. If not, you still can downgrade it to 4.1 without any issue.

 

Yes, it will install Cydia icon onto your phone.

 

So, if you want to unlock your phone again, then after jailbreak it, flash iPad baseband again with redsn0w, then install ultrasn0w 1.2.5 in Cydia to unlock it.

 

Problem may be with the SIM card. Change the SIM card or call Apple Care or make an appointment at an Apple store if there is one nearby.