Hi @pandaeyes Welcome to the Samsung Community 
Firstly verifythe Diagnostics to see of any issue is detected , Samsung Members-Support-Phone Diagnostics.
Check that the sound is at an appropriate level and microphone is free of debris as that can block sound, ensure it is cleaned thoroughly. An ideas to give your Device a Soft Reset, hold the power and Volume Down buttons simultaneously for around 7-10 seconds until it Reboots, Some times there can be an issue with wifi calling if appropriate.
However you may need to reset the Network connections, settings-General Management-Reset. can Reset the Mobile Network settings or all settings. Possibly need to ensure that the sim card is in good shape and Reset. if it is not solved though trey in Safe Mode and if problem does not exist in this mode would pinpoint recent 3rd party apps which have been installed. https://www.samsung.com/uk/support/mobile-devices/how-to-start-my-galaxy-device-in-safe-mode/
other troubleshooting is to clear the cache partition, some temporary cache files can become outmoded/corrupt. This deletes those files but your Data is intact. Follow these steps:
1. Turn off the device.
2. May need to connect To Computer , other Device or TV USB, Press and hold the Volume Up key and then press and hold the Power key
3. When the Android logo displays, release both keys.
4. Wait until the Android system recovery menu options appear.
5. Press the Volume down key several times to highlight wipe cache partition.
6. Press Power key to select.
7. Press the Volume down key to highlight yes, them and press the Power key to select.
8. When the wipe cache partition is complete, Reboot system now is highlighted.
9. Press the Power key to restart the device.
A Factory Reset is more of a last resort,would need to ensure Data is backed up before contemplating this action.
