Questions with using a Local Sim card for UBER

I understand that UBER allows you to change your Original Phone number to another number of a foreign country when you go and get a local Sim. 

How does this actually work ? If you are in your home country then you may or may not know the phone number for the Foreign country so you are in a bind. If you do know the phone number of the Foreign country you still cannot be active till you reach that country.

If you are in the foreign country and change the phone number to the local number, UBER may send a code to the phone number of the country where you set up the account - this may be a problem. Even if you are able to successfully do this one will need to change the phone number back to what it was because you will need it back in your home country. If you forget to do it then you will be in real strife because the local sim will NOT work when you get back to the home country.

Can anyone provide a detailed step-by-step guide to use a Local Sim card whne in a foreign country please.