Hi Darren,
Thank you for reaching out. Can you please provide the complete SOAP request and response logs of the API request with phone numbers and userID via reply privately to author so that I can investigate further? See here for information on enabling logging. Please make sure you have followed the phone number guidelines specified here specifically the required format of the phone number. You are correct that creating a Member of the list for phone number or userId is the same process as creating one with email.
Regards,
Mitchell
Google Ads API Team