diff --git a/fieldpoc/config.py b/fieldpoc/config.py
index 21a8609..9eae96b 100644
--- a/fieldpoc/config.py
+++ b/fieldpoc/config.py
@@ -40,22 +40,3 @@ class Config:
 
     def check(self):
         return self.dect.check()
-
-
-class ExtensionConfig(ConfigBase):
-    def __init__(self, c):
-        self.num = c[0]
-        self._c = c[1]
-
-
-class Extensions:
-    def __init__(self, c):
-        self._c = c
-        self.extensions = []
-        for e in self._c.get("extensions", {}).items():
-            self.extensions.append(ExtensionConfig(e))
-
-    def extensions_by_type(self, t):
-        for e in self.extensions:
-            if e.type == t:
-                yield e
diff --git a/fieldpoc/extensions.py b/fieldpoc/extensions.py
new file mode 100644
index 0000000..b0fac2e
--- /dev/null
+++ b/fieldpoc/extensions.py
@@ -0,0 +1,67 @@
+#!/usr/bin/env python3
+
+class ExtensionConfig:
+    num = None
+
+    callgroup_members = []
+    dect_claim_token = None
+    dect_ipei = None
+    dialout_allowed = False
+    name = None
+    outgoing_extension = None
+    sip_password = None
+    static_target = None
+    trunk = False
+    type = None
+
+    def __init__(self, **kwargs):
+        for name, value in kwargs.items():
+            if hasattr(type(self), name):
+                self.__setattr__(name, value)
+            else:
+                raise Exception("Invalid config option {}".format(name))
+
+        if self.type is None:
+            raise Exception("Type field for extension {} is required".format(self.num))
+        elif self.type == "dect":
+            if self.dect_ipei is None and self.dect_claim_token is None:
+                raise Exception("dect_ipei can't be used together with dect_claim_token in extension {}".format(self.num))
+        elif self.type == "sip":
+            if self.sip_password is None:
+                raise Exception("sip_password is required for sip extension {}".format(self.num))
+        elif self.type == "static":
+            if self.static_target is None:
+                raise Exception("static_target is required for static extension {}".format(self.num))
+
+    def check_global(self, extensions):
+        if self.type == "callgroup":
+            for member in self.callgroup_members:
+                if member not in extensions.extensions_by_num.keys():
+                    raise Exception("Callgroup member {} of callgroup {} does not exist as extension".format(member, self.num))
+
+
+class Extensions:
+    extensions = []
+    extensions_by_num = {}
+
+    def __init__(self, c):
+        for num, e in c.get("extensions", {}).items():
+            extension_config = ExtensionConfig(**{
+                "num": num,
+            } | e)
+
+            self.extensions.append(extension_config)
+
+            if extension_config.num in self.extensions_by_num.keys():
+                raise Exception("Extension num already used {}".format(num))
+
+            self.extensions_by_num[num] = extension_config
+
+        for e in self.extensions:
+            e.check_global(self)
+
+
+    def extensions_by_type(self, t):
+        for e in self.extensions:
+            if e.type == t:
+                yield e
diff --git a/fieldpoc/fieldpoc.py b/fieldpoc/fieldpoc.py
index 72f5f39..434a854 100644
--- a/fieldpoc/fieldpoc.py
+++ b/fieldpoc/fieldpoc.py
@@ -8,6 +8,7 @@ import threading
 
 from . import config
 from . import controller
+from . import extensions
 from . import dect
 from . import routing
 from . import ywsd
@@ -91,4 +92,4 @@ class FieldPOC:
         self.config = config.Config(json.loads(self.config_file_path.read_text()))
 
     def _load_extensions(self):
-        self.extensions = config.Extensions(json.loads(self.extensions_file_path.read_text()))
+        self.extensions = extensions.Extensions(json.loads(self.extensions_file_path.read_text()))