Fix

src/PhoneNumbers/Formatters/PhoneNumberFormatter.cs

@@ -50,20 +50,9 @@ protected static string FormatInternational(
         Span<char> ar = stackalloc char[arSize];
         var arPos = 0;
 
-        if (outputPrefix is not null)
-        {
-            foreach (var c in outputPrefix)
-            {
-                ar[arPos++] = c;
-            }
-        }
-
+        ar.Append(ref arPos, outputPrefix);
         ar[arPos++] = Chars.Plus;
-
-        foreach (var c in phoneNumber.Country.CallingCode)
-        {
-            ar[arPos++] = c;
-        }
+        ar.Append(ref arPos, phoneNumber.Country.CallingCode);
 
         if (charBetweenCallingCodeAndNsn != Chars.Null)
         {

src/PhoneNumbers/FrameworkExtensions/SpanExtensions.cs

@@ -0,0 +1,18 @@
+namespace PhoneNumbers;
+
+/// <summary>
+/// A class containing extension methods for the <see cref="ParseOptions"/> class.
+/// </summary>
+internal static class SpanExtensions
+{
+    internal static void Append<T>(this ref Span<T> ar, ref int arPos, IEnumerable<T>? input)
+    {
+        if (input is not null)
+        {
+            foreach (var value in input)
+            {
+                ar[arPos++] = value;
+            }
+        }
+    }
+}