The Mumbler is a caller who is completely, or almost 100% completely unintelligible, whether due to a piece of shit cel phone, standing on the wing of an airplane at 5,000 feet, a foreign accent compiled with backfiring Botox injections, laziness, or some horrible physical deformity you'd feel sorry about if you actually saw them in person. Mumblers are characterized by being super angry at not being understood, being pleased by or indifferent to generic comments that seem appropriate to whatever the hell the customer is saying, and being terrible at conversation. The benefit of a mumbler is that they will seldom require a Sup Call, normally blowing up in frustration and hanging up first.
Of course, they don't always hang up. In a fashion similar to Mary_Mosquito, some mumblers (especially ones with phones that reduce their volume to a faint, annoying buzz) will attach themselves to the phone and refuse to dislodge themselves for 20 minutes while babbling incomprehensibly about whatever the hell they're talking about.