Occasionally, when I try to use the phone (generally after not using it all night), I get a Mobile Network Not Available error on my first attempt to use it. Sometimes it happens in the middle of the day too though. I'm not sure of any correlations. I've replaced the phone....then the SIM card and the SIM card definitely helped more than anything but it still happens occasionally. Restarting/pulling the battery always seems to fix it. It's just annoying. Is this a known issue?