fix: country prefix should return string

Applied the same fix as in the main repo, but on v3.0 Vonage@ee2eec3

src/Insights/Basic.php

@@ -62,7 +62,7 @@ public function getCountryName(): string
         return $this->data['country_name'];
     }
 
-    public function getCountryPrefix(): int
+    public function getCountryPrefix(): string
     {
         return $this->data['country_prefix'];
     }