It is unlikely that the iPhone 6s was bought new 2 years ago. Apple discontinued the iPhone 6s in September of 2018, so if your phone really was "new" when you bought it, it sat in a warehouse somewhere for at least 3 years, probably longer.
That would not have been good for the battery.
Voicemail is handled by your carrier, so you would need to check with their support folks on that topic.
If the Wi-Fi on the phone is working fine at home, then you know the Wi-Fi is working correctly on the phone. Often, it is tough to get a good strong Wi-Fi signal in public because there is so much interference from other Wi-Fi networks.
As far as issues with apps.....are these Apple apps or are they apps from other developers?