TRYING TO CHARGE ME OVER 2K FOR OUTSTANDING ACCOUNT WHEN ITS A MONTH TO MONTH BILLING SERVICE!!!!! CONTACT ME ASAP

I keep getting SMSs to my number (AND my husbands number, which i never gave to you in the first place, so i would love to know where you got it from) to pay an "outstanding account" FROM LAST YEAR, apparently month to month payments is not a thing with Rain. I have been trying to return their modem to them for the last 6 months (if not more) as I dont require their services, but yet they wont collect the modem unless I pay this so called "outstanding" amount. As far as I know, if you pay you get to use the service, if not, you get cut off, so how can you tell me there are outstanding amounts????? It's actually impossible that there should be any outstanding amounts if its month to month billing. I would love for someone from rain to contact me about this and actually explain this to me!!!!!! But I dont want a call agent I spoke to one and he didn't even know what I was saying so clearly they cant help, I want someone in a managers position to contact me because then maybe they can actually figure this out and sort this NONSENSE out for once and for all. Also, another thing, I only started getting notification of this "outstanding" amount when I want to return the device. Also, how on earth did I manage to still use their services in January and pay for that month without notification of the outstanding bill and without that amount actually going to this so called "outstanding" amount. I want all my questions answered ASAP. I want this Nonsense off my name NOW!