I'll second those who suggest porting your number to Google Voice. There are occasions when institutions won't send to VoIP numbers so be mindful and make advance arrangements. Google Voice costs nothing to maintain and can be used where wherever you have WiFi or data.
I'm curious about how the application is worded. I'm (US) currently in Thailand on an METV and was only asked to choose from a drop-down list stating my purpose of travel to Thailand, from which I chose "Tourism". Beyond that I provided proof of an outbound flight within the first 60 days of the visa.
As your planned arrival is August 28, I would suggest holding off submitting the application. I've done a number of METV via Los Angeles and their turnaround is typically 4-5 business days. The visa is in effect from the date of issuance so the closer to your date of departure the better. You can start the application, just don't submit until about 7-8 days prior to your flight and after you no longer receive messages regarding documentation.