diff --git a/api/identity/definitions/request_msisdn_validation.yaml b/api/identity/definitions/request_msisdn_validation.yaml
index 018bd733..93cea2ed 100644
--- a/api/identity/definitions/request_msisdn_validation.yaml
+++ b/api/identity/definitions/request_msisdn_validation.yaml
@@ -30,8 +30,8 @@ properties:
   country:
     type: string
     description: |-
-      The two-letter uppercase ISO country code that the number in
-      ``phone_number`` should be parsed as if it were dialled from.
+      The two-letter uppercase ISO-3166-1 alpha-2 country code that the
+      number in ``phone_number`` should be parsed as if it were dialled from.
     example: "GB"
   phone_number:
     type: string
diff --git a/changelogs/client_server/newsfragments/2342.clarification b/changelogs/client_server/newsfragments/2342.clarification
new file mode 100644
index 00000000..09f4b191
--- /dev/null
+++ b/changelogs/client_server/newsfragments/2342.clarification
@@ -0,0 +1 @@
+Clarify the expected phone number format.
diff --git a/specification/client_server_api.rst b/specification/client_server_api.rst
index d1d5dee4..7e420b8c 100644
--- a/specification/client_server_api.rst
+++ b/specification/client_server_api.rst
@@ -1042,6 +1042,9 @@ wishes to canonicalise the phone number, then it can use the
     "phone": "<The phone number>"
   }
 
+The ``country`` is the two-letter uppercase ISO-3166-1 alpha-2 country code
+that the number in ``phone`` should be parsed as if it were dialled from.
+
 Login
 ~~~~~