It goes to the same place as the calls, route the calls and they will be routed to the same destination.
If you point the DID to an IVR, look in the IVR settings (in the WebClient > admin > Call handling > new IVR ( or Edit existing ) > Options > Forward SMS ).
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It is no longer necessary to activate. As soon as a DID of a Trunk with SMS capabilities points to a user or a Queue where the user is a member .. it is automatically activated and allows him to be able to send SMS.
If the user can receive SMS from several destinations (DID), when he tries to create a new conversation, after write destination number, the list will be displayed and he will be offered to choose which DID he wants to use for this conversation.
Me, I find it good, before, it was not possible to choose from which DID we wanted to send the SMS ... it was only one possible DID to create a new conversation (It was possible to answer a conversation to another DID but not initiate ). Now it is possible to choose the DID to use, individually, for each conversation.
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The guide you seem to be following is for version 18.5, I believe.