diff --git a/frontend/src/Datarekisteri/Frontend/FormFields.hs b/frontend/src/Datarekisteri/Frontend/FormFields.hs
index d360d9c..7121fc1 100644
--- a/frontend/src/Datarekisteri/Frontend/FormFields.hs
+++ b/frontend/src/Datarekisteri/Frontend/FormFields.hs
@@ -39,7 +39,7 @@ telephoneField = Field
     , fieldView = \id name otherAttributes result isRequired ->
         let result' = either (\x -> x) renderPhoneNumber result
         in [whamlet|
-               <input type="tel" id="#{id}" name="#{name}" value="#{result'}" *{otherAttributes} :isRequired:required="true">
+               <input type="tel" id="#{id}" name="#{name}" value="#{result'}" pattern="[+ 0123456789]*" title="Only '+', spaces and numbers are allowed" "*{otherAttributes} :isRequired:required="true">
            |]
     , fieldEnctype = UrlEncoded
     }