You can still back up the Mitel SX-200 with ProComm Plus if you are remote. I am assuming that you have remote access.
Set up your Procomm for VT-100, 9600, N-8-1, ASCII.
After you have dialed the remote access modem number and Procomm confirms that you are connected, Press enter 4-5 times to wake up the maintenance terminal on the SX-200. Choose 1 for VT-100 Terminal. Then Choose 1 for Maintenance. You will be prompted for a user name. The user name will be INSTALLER, MAINT1, OR MAINT2. After entering in the user name, you will be prompted for the password, hopefully you have that.
Once logged in, you would choose the softkey 1 for SYSTEM. Then Choose 3 for Database. Then Choose the softkey 1 for BACKUP. Then 1 for Kermit. Press 0 or enter. You will then be prompted for a file name, keep it short. (you will find the file after the backup is done in the Download folder of Procomm Plus.) You will then be prompted to CONFIRM you action. Before confirming, I like to change my Procomm setting from ASCII to KERMIT. Then I confirm. You may have to click on the "RECEIVE FILE" icon to start the transmission. Backups are typically about 5 minutes.
Hope that helps!