1. This can be caused by poor/loss of signal withing the area. Try turning the phone off, waiting a few minutes, then turning back on and it may update the PRL if an update is needed. Another option is to turn off the phone, remove the SIM card, wait a few minutes, then reinsert the SIM card, turn the phone back on, and let the phone establish a fresh network connection.
2. It you have just got switched to LTE, there may be a glitch within the network servers (in wich case you will have to contact your carrier - your LTE simcard may even need to be replaced).
3. Under Settings > Call Settings > DDTM/similar setting "Send voice calls to voicemail during active data connection - make sure that is unchecked.
Having said all, the most precise walk through can be received from one of your carrier's Tech Support reps as this issue tend to stem on network systems glicht - and its troubleshooting and resolving may sadly drag a little longer due to its nature.