The emoji sequence represents the secret key exchange between you and the other party. If you intercept the call, you are making one key exchange with person A, and another key exchange with person B. Due to the mathematics involved, there is no way for you to force both key exchanges to yield the same result.
For a "standard" DH key exchange it would be possible to brute force the emoji sequence to be the same (since it's too short to be resistant to brute forcing), but the protocol that Telegram uses specifically defends against that by having both sides commit to their share of the key ahead of time, so they cannot try different numbers.
So person A and person B are going to see different emojis no matter what you do. To fake a phone verification while performing a main-in-the-middle attack you'd also have to fake their voices to each other. That's hard.
For a "standard" DH key exchange it would be possible to brute force the emoji sequence to be the same (since it's too short to be resistant to brute forcing), but the protocol that Telegram uses specifically defends against that by having both sides commit to their share of the key ahead of time, so they cannot try different numbers.
https://core.telegram.org/api/end-to-end/video-calls#key-ver...
So person A and person B are going to see different emojis no matter what you do. To fake a phone verification while performing a main-in-the-middle attack you'd also have to fake their voices to each other. That's hard.