diff --git a/sinch/domains/voice/models/callouts/requests.py b/sinch/domains/voice/models/callouts/requests.py
index 2efe21be..753ce2f5 100644
--- a/sinch/domains/voice/models/callouts/requests.py
+++ b/sinch/domains/voice/models/callouts/requests.py
@@ -9,7 +9,7 @@ class TextToSpeechVoiceCalloutRequest(SinchRequestBaseModel):
     destination: Destination
     cli: str
     dtmf: str
-    domain: str
+    domain: Literal["pstn", "mxp"]
     custom: str
     locale: str
     text: str