SMS messages will use the client device carrier to send messages, there is no place to set this manually. You can send out PPSK credentials that you manually create yourself, or you can set up self registration to allow guests to register themselves when they come in to the network, or you can set up self registration with Employee Approval, so that guests can register but you or an employee will have to approve the guest before they can log in. I've included some guides for you below, these guides assume you are using HiveManager (formerly NG), if you are using classic there are guides available for these functions, but they would be different.
PPSK SSID- https://thehivecommunity.aerohive.com/s/article/PPSK-SSID-in-NG
PPSK Self Registration- https://thehivecommunity.aerohive.com/s/article/Self-Registration-PPSK-SSID-in-Hivemanager
PPSK Self Registration with Employee Approval- https://thehivecommunity.aerohive.com/s/article/Employee-Approval-with-Self-Registration-SSID-in-NG
