2 year return plan

I was a VERY long standing client of Telus, and my last contract with them was a 2 year return plan. The 2 years have come and gone, and I returned the phones to Telus. They in turn charged my account for the remaining amount due for the phone and sent me a third party Mastercard credit in the same amount charged to my account. So far so good. Problem now is the following: I try using the prepaid Mastercard THEY sent me to pay my outstanding balance on my account and is does NOT work. I am suppose to go back to a store and see if Telus can speak to the third party company and replace my credit card. Nothing is easy, and I am supposed go around their problems and create an inconvenience to my for something they created. Anybody else have a similar problem? Very frustrating !!