diff --git a/rust/tests/4_create_phone_number.rs b/rust/tests/4_create_phone_number.rs
index c4ec151..d492a89 100644
--- a/rust/tests/4_create_phone_number.rs
+++ b/rust/tests/4_create_phone_number.rs
@@ -1,6 +1,19 @@
+const PHONE_MASK: &str = "(___) ___-____";
+const ZERO: u8 = '0' as u8;
+
 fn create_phone_number(numbers: &[u8]) -> String {
-    let numbers = numbers.iter().map(|i| i.to_string()).collect::<String>();
-    format!("({}) {}-{}", &numbers[..3], &numbers[3..6], &numbers[6..])
+    let mut position = 0;
+    PHONE_MASK
+        .chars()
+        .map(|c| match c {
+            '_' => {
+                let c = (numbers[position] + ZERO) as char;
+                position += 1;
+                c
+            }
+            c => c,
+        })
+        .collect()
 }
 
 #[test]