Hi there!
The timestamp shown next to the phone number value in your phone attribute is just a visual display, and Customer.io does not treat the data any differently.
The timestamp appears because the number fits the UNIX timestamp format, which can be useful in cases where you need to see the human-readable date it translates to.
To avoid this issue, it is recommended that you use the E.164 format, which is the international standard phone number format. This format includes a country code, area code, and subscriber number and prevents the number from fitting the UNIX timestamp format.
You can learn more about the E.164 phone number format in this link: https://customer.io/docs/twilio/#add-phone-numbers-to-people-in-customerio. The phone number attribute value is still just a string and treated the same way as other attribute values.
I hope this helps! 😁