Cellphone Network problems

Today I got scared while going to work in seeing that i had no network on the phone.
My girlfriend's phone had no problems (she has the same network) and after reboot (engineer solution) it remained the same.

So when testing for solutions is that I got me worried. I tried Automatic network search, and after 2 minutes of processing, the phone began a restart loop...

So I tried to change SIM card with my girlfriend. In her phone, all fine.
In mine all the same...

But I found that without a SIM card, my phone had no problems...

I went home to back up my contacts and because i left there my USB cable. Upon arrival, I noticed that my phone already had network and everything worked fine. :0

Upon returning to work, I lost network again. So that was the problem, since my girlfriend's phone always worked?

Well, here's the secret:
I only needed to change the Network Mode of 'GSM / WCDMA Automatic' to 'GSM only'.

I assume that my carrier has a problem with the 3G in the area of my job from now on, as at home it works well on 3G and GSM works fine at my job.

I hope to help someone with this ...