Each SMS message has an associated sender ID, which is an originating phone number or string identifier used to indicate who an SMS message comes from. Members of your audience subscribe (opt in) to each sender ID they want to receive messages from.
Alpha codes have become a common sender type for SMS marketers throughout the world, however they are a one-way sender, so cannot receive inbound SMS (mobile originating, or MO) messages. This can be a problem for brands required to maintain regulatory compliance with SMS keywords. For example: STOP, HELP, CONTACT.
You can maintain compliance by forwarding messages from a two-way sender to an alpha sender.
See SMS Senders to learn about the various sender types and how message forwarding works for alpha senders.