Two users are required e.g. Retail Clients - user1 & user2. We need to add two numbers in the Dialing Plan,e.g: 888111 and 888222 (don't forget to choose the proper Route type and Route).
You can register both users in Vippie or a typical ATA phone like Yealink T-10.
To send SMS to user2 just enter the number 888222 and write a message.
To send SMS to user1 enter the number 888111.
This scenario was tested on Voipswitch v. 2.985.0.86. Both Yealink T-10 phones had firmware 3.0.1.68 and support SMS sending feature.