I've only used Uber here in the US a few times but that was 3-4 years ago and from what I remember, there is a generic phone number you call and it's based on ANI (Automatic Number Identification) since you have to call that number from the same phone number the account is registered under and it will route your call to the driver. If you try to call from any other phone number, it says something to the effect of sorry, but we are unable to locate you so its similar to how it works in Shanghai. Never saw it another way as this was also how it worked for Lyft and Sidecar too.