If you don’t have a POA arrangement, then you don’t have the authority to act on that other person’s behalf, it may seem unreasonable if your intention is honourable but that’s the way it is .
You could (depending on circumstances of the individual) get them to call , clear the security/identity confirmation etc , then they state you are authorised to act on their behalf and you continue with the conversation to presumably cease the services at their previous address .
