Q: why does my iPhone switch from the bluetooth to the device when making calls

Since getting my Iphone6+ I am having problems keeping the connection to the Bluetooth earpiece.  I will input the number and hear it in my ear but as soon as I hit send the phone switches the audio to the phone.  I can open audio and select the device again it will connect and I can hear it ringing  sometimes it will switch it back again and I have to hit the device again to get it back in my earpiece.  I have so far tried  forgetting the device and then re-learning it but it still continues to do it. Thanks for any suggestions.