Why is my cellphone sound garbled?

It is an Android Galaxy Ji6 phone that is two years old. I got for $150 and $45 month subscription. Only long-distance phone calls are not working.
Sometime it works most of the time is garbled.
Should I buy a new phone maybe? What do you suggest? Only long distance phone calls are not working perfect.
I have two ISP . Both cable and internet. One of the internet connection just got signed up last month.

I don’t have room for updates..