If you need to use ever changing phones in foreign countries, then use Traveltel. Rates are much higher than callback. Rates from Switzerland , USA, everywhere . A callback Telephone phone company saves you money on long distance and international phone calls. You circumvent local phone companies and get cheap phone rates from a USA based company.
Callback discounted rates are especially interesting for residents of other countries.
You need to sign up for a callback company. For this you should have an international credit card. Once you have signed up, you get your personal Trigger number. In seconds, you receive a call back to your phone and get a dial tone. You dial as if you were in the US, you will receive detailed instructions as to which prefixes to use You can program the callback procedure into your telephone .