There may be an easier way to do this, but you can setup Office Communicator to "Save my instant message conversations in the Outlook Conversation History folder", which will save all instant message conversations you have and it will also e-mail you when you miss a message, with the subject "Missed conversation with". You could then setup a rule through Outlook to forward these messages to your phone e-mail or depending on your service provide it may be able to e-mail you a text message.
Office Communicator configuration:
1. Go to Tools > Options.
2. Under the Personal tab, there is a check box to enable "Save my instant message conversations in the Outlook Conversation History folder".
1. Tools > Rules and alerts.
2. New Rule.
3. Start from a blank rule > Check messages when they arrive, click next.
4. Check "with specific words in the subject, then click "specific words" and type "Missed conversation with", click next.
5. Check "forward it to people or distribution list", then click "people or distribution list" and specify the e-mail that you would like to send the message to.
That's the round about way that it could work, assuming you can't already check company e-mail on your phone.