Short Code and Long Code (Virtual Mobile Numbers) is called 2 Way SMS solution (VMN) to which your clients or members can send a text SMS starting with a keyword. After accessing our VMN application, you will receive a number on which you will receive messages which can also be forwarded to your server or database. 

Long codes are standard phone numbers like +1 (407) 555-0100 or +44 7700 900396. 

Short codes are 5-6 digit numbers, from which you basically receive promotional SMS like "Type KBC<space><A/B/C/D> to 55647' to answer your question for KBC". Here, KBC is the keyword.

You should have credits in your Inbox balance to receive the customer replies to your VMN.  If your account has 0 credits, then you might not receive messages through the keyword.

Format of the keyword should be correct while customer replies on your VMN.