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Switch 2's GameChat feature requires a mobile phone number
Those wanting to utilise the Switch 2's GameChat feature will need to register a phone number.
However, if you read the small print, users will also need to register a phone number with a Nintendo account to access this feature. On the same page, the company states that as an "additional security measure" text message verification "is required to set up GameChat". It adds that text message verification for children wanting to make the most of the Switch 2's GameChat feature will need to use a number registered to a parent or guardian's account to get access.
Or read this on Eurogamer