@Adviuz Hello again. I do not have an answer for you, just ideas to play with:
(1) Use a match() function to search for the phone number among all the phone numbers previously entered. Match from the beginning of the column to the cell just above the last phone number.
(2) Wrap the match() function inside an iferror() function: if the match() function returns a number then it means you have a duplicate, if there is no match then have the iferror() function return 0 (for instance). 0s will indicate you have a new number, any other positive integer will indicate a duplicate.
(3) This being said, the two points above imply that the latest phone number was submitted and then only tested. This is not your use case, since you want your phone number to be tested first, then submitted or refused.
Hopefully someone else will be able to help you along further.