[iicc]

GDPR Article 16 gives you the right to have inaccurate personal information amended, taking in to account the purpose of the information.

A phone number used for this purpose would definitely identify a person, and so qualify as personal information.

[ghaff]

Presumably you need to prove who you are to make changes to your record however. GDPR doesn’t give me the right or require a company to change a phone number if I call them up and tell them to change the number for some random name without proof of identity.