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Hambrise,

any of the phone numbers belonging to account can be used as a username. It can be not only number belonging to particular extension but also main company number if a user provided extension number along with main number to login. It can be also email address. So I don't see much sense of displaying particular phone number which was used to login. In our own sites we usually display first and last name of a user and also main company number and user's extension number.

Make an API call to "/restapi/v1.0/account/~/extension/~" 

"~" means current authorized user.

If you need the associated phone numbers with the user, send an API call to "/account/~/extension/~/phone-number"

Ref:

https://developer.ringcentral.com/api-docs/latest/index.html#!#RefGetExtensionInfo

https://developer.ringcentral.com/api-docs/latest/index.html#!#RefExtensionPhoneNumbers.html