The failure you met does not possibly depend on the phone. The connection of network is necessary to make a good call. So this kind of problem also depends on the circumstance. For example, in the noisy place or all building and underground channels, the signal is easy to be interfered and shielded. It's common if it is not clear to make a call in such circumstance. You can go to another place to make call and it will be okay. However, if it happens in different areas every time, then you should take your phone to our service shops to have it examined.