For Guest Portal, one of the authentication options is Authentication code via Text Message. The authentication code can either be sent via a free method via the cell provider, or a paid aggregator such as Twillio.
For the free option through the cell provider, this relies upon Email to SMS. We have begun to receive reports of guests not receiving the authentication code intermittently or not receiving at all for some of the cell providers. We have received feedback from some of the providers they are deprecating or enforcing limits on the Email to SMS service, and they recommend moving to leveraging aggregator services. We are aware of intermittent issues with T-Mobile and Verizon.
Based on the feedback from the cell providers, if you have a large population of guests that are T-Mobile or Verizon customers, you may wish to investigate using a paid aggregator service such as Twillio or Broadnet.