Do you want to change something in the customer center or in the community?
Is the contract displayed in the customer center a mobile phone contract?
Do you want to change this number or just the contact telephone number?
In both cases, I suggest that you report this directly to Swisscom. Then you can be sure that the number is stored correctly.
But if you mean the Swisscom Community, you cannot store a telephone number at all. For the login you have to make settings in the customer center.