Myphone Hammer Energy Phone & Plantronics Multipoint Explorer 50: Bluetooth Connectivity Issue

Hello.
I have a Myphone Hammer Energy phone and for this I purchased the Plantronics Multipoint Explorer 50 bluetooth headset.
When I pair my earpiece with my phone and bluetooth is on, everything works. When I turn the handset off and on, everything is fine too.
The problem appears only when I turn off and turn on the bluetooth option on the phone. Then the phone is connected to the handset, but the sound does not go through the handset anymore, but normally through the phone.
Anyone had a similar problem? If so, please help.

First, do a factory reset on your phone. i See if the problem persists