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My apologies for delayed response to your question.
I think with the current account settings and configuration, there is no option for doing that. But if you want to work-around the limitation, then my suggestion is to write a small script, listening to incoming SMS message to the main company number, then forward (send) the message to all extensions which should receive the SMS message.
If this solution is good enough and you would like to have some code snippet, let me know so I can help.