Where the antenna is located inside the phone is not an issue. It's where people expect their phones to work is the issue. (inside tunnels, metal buildings, etc.). Radio waves simply cannot go through earth and steel very well.
Also, atmospheric conditions can create natural signal drifts at times. This can and will happen to any phone.
The fact that the antenna is built inside the phone isn't much of an issue, either. Cell phone frequencies are much higher than FM, so the phones don't need a very long antenna to work properly, unlike FM radios.