Q: iPhone 5C Call Quality Issue

So my mom got her iPhone 5C, A T-Mobile unlocked phone that my mom is using under MetroPCS and was bought on eBay, about a week ago.  She activated it like a week ago and so far she's been in love with it. But about the 2-3 days later my dad tries to call her but he hears this weird staticy, water spashing sound in the background and I've heard it too. She's also called internationally and my other family member also hears it too. This wasn't a problem at all before she got it because she had called the family member and it didn't do that at all, and she was on the phone for about 20-30 minutes. And I've already tried to restart it and updating iOS. Is there anything else I can do or will I have to go talk with the seller to maybe get me another device? Thankfully the seller is giving the buyers a 6 Month warranty on the device.