diff --git a/modules/clan/phonebox/default.nix b/modules/clan/phonebox/default.nix
index 8d30da3..5cb737d 100644
--- a/modules/clan/phonebox/default.nix
+++ b/modules/clan/phonebox/default.nix
@@ -79,7 +79,7 @@
                   ln: builtins.concatStringsSep "" (builtins.genList (_: "X") (builtins.stringLength ln));
               in
               ''
-                exten => _${prefixNumber}${replaceWithX localNumber},1,Dial(PJSIP/''${EXTEN:1}@${hostname},30)
+                exten => _${prefixNumber}${replaceWithX localNumber},1,Dial(PJSIP/''${EXTEN:${builtins.toString (builtins.stringLength prefixNumber)}}@${hostname},30)
               '';
 
             getYggdrasilIP =