Can I complete my return without a US phone number? I am unable to speak with the assistant by phone.

I still don't see why the IRS needs your phone number on a tax return, since they will never call you. Basically, if you just leave it blank, you'll be fine.

A foreign phone number cannot be entered.  A phone number is not required to file a tax return, however TurboTax requires a phone number to be entered.  You can enter any number with a valid US area code, such as (203) 222-2222,