I AM NO LONGER USING THE MOBILE NUMBER YOU HAVE ON FILE, I NEED TO UPDATE IT

I have been sending email to payflex regarding my account, I need to update my mobile number but I am unable to do so due to me not using the registered number on their records, the OTP is sent to the registered number and I am no longer using that number, I have tried to call them but the phone ring but no one answers, sometimes calls will be dropped. I do not know how many emails I have sent to them but explaining my problem but no help. Very disappointed by their customer service, it is so poor and I will never recommend them to anyone