More information.
Seriously, that could be a damaged phone, a software problem, a network problem. So what phone, what network, has it just started, did the phone work properly before this, has anything else changed (new SIM, software update), has the phone been dropped, has the phone ever got wet, does it happen for all calls or just some, does it happen everywhere or just in some places, does it work if you put the phone in speakerphone mode.
If we have more information we've a chance of identifying the cause, and hence maybe suggesting a solution.