diff --git a/README.md b/README.md
index 058aace..a873db4 100644
--- a/README.md
+++ b/README.md
@@ -16,6 +16,7 @@ Enable at least the following scopes:
 * `sessions:fax:write`
 
 Enter the token-ID as `SIPUSER` or `TOKENID` and the token as `SIPPWD` or `TOKEN`.
+It's prefered to use the `TOKEN*` variables, but in that case, the `SIP*` variables **MUST** be left empty/unset.
 
 If you use Letterheads Address database, uncomment the last line of that file and add `your/path/to/Letterhead`