A constraint on the second field to check it being equal to the first should do what you need. You'll probably want to add a regex related to your phone number parameters (example discussed in Set Limit On Integer). Also, be sure to include a helpful constraint_message.
number_match.xlsx (8.7 KB)
+------+---------+----------------+--------------+------------+
| type | name | label | constraint | appearance |
+------+---------+----------------+--------------+------------+
| text | number1 | First number: | | numbers |
| text | number2 | Second number: | .=${number1} | numbers |
+------+---------+----------------+--------------+------------+
