diff --git a/sysmocom/suites/nitb_ussd/assert_extension.py b/sysmocom/suites/nitb_ussd/assert_extension.py
index e547131..c0c32be 100755
--- a/sysmocom/suites/nitb_ussd/assert_extension.py
+++ b/sysmocom/suites/nitb_ussd/assert_extension.py
@@ -34,4 +34,4 @@
 
 print('Sending ussd code %s' % USSD_COMMAND_GET_EXTENSION)
 response = ms.ussd_send(USSD_COMMAND_GET_EXTENSION)
-assert ' ' + ms.msisdn + '\r' in response
+assert ' ' + ms.msisdn() + '\r' in response