Why can't I receive calls in LTE

I'm having problems with my iPhone 6. I had similar problems with my iPhone 5s. When the iPhone has an LTE connection, and is stationary, very often I cannot receive calls. Individuals calling me hear no ringing, there is no forwarding to voicemail, and I have no indication via text message that someone tried to call me. Also, awkwardly, when testing this problem, always the third attempt when calling my phone from a landline results in the call going through. Very often where is a delay receiving SMS/txt messages. When switching to 3G, there seems to be no problems. Its almost as if the iPhone freezes or refuses to switch fast enough from LTE to 3G to pick up a call.

During a test, if the phone is LTE, and I call from a landline, there is no ringing tone either in the landline phone or on the iPhone. When I switch during such a call to 3G, I hear a busy signal in the landline. I am on a network in Europe that is not certified by Apple, but is by far the best LTE network in the country. All the network settings are as the network recommends, and all other aspects of the service are tremendous, except for this problem.