Monotronics International SCAM! Takes Advantage of Elderly! Dallas Texas

My elderly, single mother had Monotronics security system installed years ago.  In June, 2010, she called Customer Service to cancel her contract because she didn't even use the system since it was too difficult for her to understand.  They informed her to send in a written notice of cancellation, which she did on 6/17/10.  They continued to bill her & not realizing who she was even paying, she continued to pay.  Now she has another company install a new, more managable system for her & she called Monotronics to see if they want the old equipment back.  They informed her that she still has 3 more years on the contract that she MUST pay.  My mother never even realized, nor could she read, that she even had a 5 year contract with them!  They are insisting that they never received her phone call (which I KNOW for certain they did) nor her letter of cancellation (which I took to the post office, myself)!  How convenient?!  They are refusing to allow her out of this last 2 years on the contract.