iPhone 5S speaker/mic issue

once in a while, the mic on my phone just cuts off during a conversation. i can hear the caller perfectly fine but the caller is unable to hear me.

also, when i play voicemail messages using the speaker, i can barely hear it even tho i have the speaker volume at max. i have to reopen the phone app just to clear out that bug.

anyone having this type of issue with the iPhone 5S speaker/mic? this only happens once in a while