Unsuccessful calls from Macau to China in cell phone

I dial 00 + 86 + cell phone. In my first calls, my call was received and I could call. Sometime later I dial in same way, i hear the waiting music of the dialed phone, but never again my friend could receive my call. I check through chat in a site that my friend have the phone free, but did not receive any sign of my call.
This is happening with 3 different friends in China. My phone company have no explanation for that.
Can u have?