Opened the website with Burp Suite.

Added some random numbers and clicked sumbit.

In the reqeuests i changed the numbers to the actual phone number.

I got the flag in the response headers.

Flag: `bcactf{PHoN3_num8eR_EntER3D!_17847928}`

Original writeup (https://github.com/juke-33/Write-ups/tree/main/BCACTF%205.0/webex/Phone%20number).