Windows Mobile Standard

Hi, I have an unusal issue that makes me nervious. I have this problem about a week because I own it just a week. Sometimes, I don't know why, I can't get any call from others(I have signal and such, looks like its online with my carrier). This occures just a couple of times and its very disappointing because I like the phone. Can help me someone with the problem? I would apprecitate it,
bye