To be clear, this question is not concerned with surveillance by the NSA or other government authorities.
If you are just dealing with the average hacker than listening to normal calls is not a problem. Communication within the mobile network are encrypted and although the encryption is weak enough for government it is still strong enough for the average hacker.
But note that the not-so-average hacker can obtain or build stingray like devices and thus intercept and decrypt the communication.
Or does simply owning a smartphone create more problems than it solves?
Given the amount of information the applications on the smartphone collect about you for monetizing it might be better for the average user to stick with a dumb phone than to use a smart phone. This is at least true for the Android phones where a large part of the phones has known vulnerabilities and some of these are actively exploited. It might be different for the more closed and more tightly updated iPhone but malware is known there too.
The situation might be different for a not-so-average user who is able to harden the phone and make it do only the things the user actually needs or for the user buying pre-hardened phones. In this case better encryption found in various messaging and phone application might actually provide more security than the usual encryption of mobile communication. It still leaves the problem what information are available for the hacker by simply stealing the phone. These data are usually only protected by weak authentication, i.e. fingerprints which can be faked, pattern are often not that hard to find out too and passwords can often be guessed or are known by should surfing.