my iphone 6 keeps switching between silent mode and audible all by itself

I have an iPhone 6 of 64GB storage and 8.2 version of iOS installed. I have the problem that my phone keeps switching between silent mode and audible all by itself. It happened quite randomly in quite a lot of situations when I use my phone. I have tried to restarted, reset my phone to troubleshoot but it just happened again. I have sent my phone to the authorised Apple service centre in Singapore (A-Lab) for hardware checking but eventually they told me that there was no problem at all and just returned my original phone without doing anything. Now I still encounter this annoying issue quite frequently as sometimes it just happened in a row. what should I do now? Thanks.