I bought this phone 2 months ago, but from time to time, I was experiencing this issue: the phone prompted an error message saying "Not registered on the network" when I was trying to make an outgoing call. I had this problem again this evening, and I can't make a call with this error message. I can see the signal bar, but the HSDP (the H icon by the signal bar) disappeared.
This issue was not happening in the same place, and when I was traveling to Vegas last month, I had this problem as well.
So I was just wondering if it is a problem of my phone, or due to some network coverage problem? How can I fix it?