SMS and email connectors at Tools → Integrations → Notification.
Tools → Integrations → Notification (/tools/integrations/notifications) lists SMS / email (and similar) connectors. Open a card for /tools/integrations/notifications/{connectorId}.
Who can open it: users granted Notification (and the integrations feature).
The detail page has Connect (credentials / presets) and Send a test.
Pick a preset if offered, paste keys, save.
Use Send a test before you rely on invoice email or OTP.
Old SMS menu URLs redirect here.
Invoice email button disabled: this connector is off or missing credentials.