diff --git a/fieldpoc/extensions.py b/fieldpoc/extensions.py
index b0fac2e..0de5902 100644
--- a/fieldpoc/extensions.py
+++ b/fieldpoc/extensions.py
@@ -34,6 +34,10 @@ class ExtensionConfig:
                 raise Exception("static_target is required for static extension {}".format(self.num))
 
     def check_global(self, extensions):
+        for e in extensions.extensions:
+            if e.num != self.num and e.num.startswith(self.num):
+                raise Exception("Extension {} uses {} as prefix, which is already defined as an extension".format(e.num, self.num))
+
         if self.type == "callgroup":
             for member in self.callgroup_members:
                 if member not in extensions.extensions_by_num.keys():