iphone 6plus 64gb MGAK2ZP/A cellular and wifi problem.

hello, I am using iPhone 6plus 64 gb, model no MGAK2ZP/A, I have problems with wifi and cellular connections, when I connect to the cellular network to 4g some times it will work great but some times it will show the symbol 4g and the surfing of the data is also fast but the voice calls or video calls through face time or whats app is very slow, some times even it does not connect the call to other person over 4g, I thought that it was network problem but I checked with my friends iPhone 6plus with the same network inside his handset, and if sometimes cellular network works fine then wifi will not work and if cellular network is not working fine then wifi works great. I tried everything by resetting network settings and erasing and resetting and resetting iPhone as a new device but still my problem exists. Is it a software issue or a hardware issue. Friends, help me please, thanks in advance.