Why does the smartphone catch the network, but calls do not go?

Question

Mobile devices

Hello.
I have a samsung galaxy s2, recently I began to notice that the phone stopped working at my workplace. The network catches, but the calls do not come and I myself can not call. 3G is not working. It is necessary to move away from the workplace, somewhere to the window or to the street, then everything starts to work.
Reboot saves for 5-10 minutes.
Firmware cyanogenmod 10.1.3.
Region: Moscow.
Nobody had this?