The iPhone may be great to have, very slim, already has issues related to the network but Apple is on its case. Every mobile has its drawback, all the iPhone’s ever created have had this problem but they all have solutions in the end. The iPhone’s have been having problems where your voicemail is activated but the call continues to ring and ring. If you finally get there and want to get to the last message, better have patience, you can skip.
A simple solution to it would be to send it to Apple or the store you bought your iPhone from. If you bought it privately, then send it to one of the network providers, they should be Apple certified.