maintaining a US phone mumber for verification codes is well nigh impossible.

You don't need one, as I know now after trying since 2h to open an account at IB.

Situation: I have German phone number. I am currently in New Zealand. The first steps of creating the account did work, so it does exist. I can log in with username+password and the verification code is sent to my German phone no. (while I am here in NZ; works; no problem).

The "trying since 2h" is because at the probably very last step of setting up the account (agreeing with lots of disclosure docs and signing) after "continue" I get always thrown out with "...server error or inactivity".

Logging in again (verification code again sent to my German phone, no problem), trying with a different browser, different device - same error at the same step.

Calling them, being as expected asked by both I talked with: "What Browser?". My reply: "First with my tablet, first with Firefox, then with Chrome. Then with my Win10 laptop, first with Edge, then with Firefox --- and, yes, always with cookies enabled, any add-on deactivated, scripts enabled...". That makes them go quiet, because their standard advice of course would be "Try another browser...".

I am close to give up, having lunch now to mentally regenerate - before trying again.

Back to your US phone no. issue: I can assure you that's not needed as in said process for those dozen of logins I received the same number of verification codes from IB on my German phone.