Cubot X20 Pro bad call quality

I purchased the phone 3 months ago but have had olts of problems when people phone me and after 3 /4 mins the quality of the call deteriorates in that the person cant hear me or it breaks up, the sim worked ok in my samsung galaxy, i have been to Network supplier 02 and they say the sim is fine ? can anyone help....I have also tried to contact via email Cubot UK but no one answers back ...pls help