If you are not comfortable with phone confirmation then use the send pin via post option that would be received by client as it would not be a good option to verify with different number. You might have to verify again if the number is changed.
we call the client and ask for the name of the person most likely to NOT throw the Google postcard away. We then speak to that person and explain the process. Now, we initiate the mail option and wait 2 weeks. After two weeks we begin calling the client each week until they confirm receipt.
Not streamlined but it works - we do 4-5 of these a month.