Unable to change my own debit order banking detail

I contacted the call centre 084111 four times only to find out i cant change my details because im call from a prepaid cell number. Then i contact a service online and chatted with online agent by the name of loyld. He then transferred me to another department because my 2 contract numbers have been moved over to another system. He transfers me and i hold on till the call cuts. i then try calling *** and they transfer me again with the same story that my contract numbers are on another system. I call again the agent then tells me the system is off line, i asked the agent can i go into the store to change it as all i want to do is change my banking details to another account. Its shocking that i had to make so many calls and still not get anything change also to drive to a branch only to tell me we can assist you. Why is this so difficult to change im being honest by making these changes as the other account will no longer exists. But your company cant help me.