Storing User Phone Numbers in a Voice Assistant: Solutions Needed

I'm building a voice assistant where the user says their phone number. I want to store it in {{phone}} and reuse it later (e.g. in a webhook like /api/check-phone?phone={{phone}}), but on the server side, it always comes through as undefined. How do I make sure the captured phone number is saved into {{phone}}, or is there a default system variable I should use instead?