iPhone 5SE ringtones stop working! Help?

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Hi. I know how to d/l and set ringtones on my newish iPhone 5 SE. However about once a week, the ringtone flat stops working and the phone seems to default back to some crappy, tinny sounding notes melody. I can then reset the ringtone and that may or may not work. If it does, it will stop again in about a week. I've tried about 15 different ringtones from the Verizon ringtone store. Same same. I'm talking about when someone calls me, and the phone rings using the ringtone I set. Any help will be appreciated! Thanks!  Kathy on the Outer Banks of NC