Onoff is a mobile app that gives a smartphone one or more additional phone numbers, so users can make and receive calls and text messages on a second number without a second SIM card or device. The app runs on iOS and Android and includes a custom voicemail and contact synchronisation. It is aimed at people who want a separate number on the phone they already use.
Onoff Telecom SAS is registered in Paris, and its privacy policy names its registered office in Paris, so the company is headquartered in France. The policy states that some customer data is passed to providers outside the EU, naming the United States, under the European Commission's standard contractual clauses, and it refers to the GDPR. It does not say where primary storage is located. The site describes no open-source components or self-hosted option. For a European organisation, the main point to check is the US sub-processing covered by these clauses.