You can install the Num2 app on a device with an Israeli number, and on a device with a foreign primary number in the US and many European countries. The list of countries is updated from time to time, you can see the latest list at www.cellact.com .
If a subscriber travels outside the country to which the main number belongs, they will be able to send and receive SMS if the device has internet connection, even after changing SIM to a local card.
Receiving and making calls will work fine for users who have purchased VOIP packages, provided there is a stable internet connection. Also, receiving and making calls from the Num2 for users without a VOIP package, will only be possible if the primary number has an abroad, without a password. Proper call service depends on the roaming settings of the overseas host operator and the subscriber network operator.