Who else to contact?

I have been trying to change my phone number for the past 3 months. I can’t log in to my journal because I need the security code which goes to my old number. I have contacted the helpline who have said it is the job centre who will need to do it. They have repeatedly left messages which the job centre are acknowledging but no one is getting back to me. I asked for a phone call rather than a face to face appointment which they have said is fine to request due to my health conditions but for three months no one is getting back to me. Not even to book a face to face appointment. I tried contacted my MP but no response. I am just not sure what else to do now. All I want to do is change my phone number.