diff --git a/sysmocom/suites/ussd/assert_extension.py b/sysmocom/suites/ussd/assert_extension.py
index 87f2ee5..11a5c5b 100755
--- a/sysmocom/suites/ussd/assert_extension.py
+++ b/sysmocom/suites/ussd/assert_extension.py
@@ -47,4 +47,4 @@
 print('Sending ussd code %s' % USSD_COMMAND_GET_EXTENSION)
 response = ms.ussd_send(USSD_COMMAND_GET_EXTENSION)
 log('got ussd response: %r' % repr(response))
-assert response.endswith(' ' + ms.msisdn)
+assert response.endswith(' ' + ms.msisdn())