Over the last 6 months, my old Treo 600 touch screen was dying. Cleaning and re-seating helped for a while, but eventually, the touch screen stopped working. As the Treo 680 barely improves on the Treo 650, and costs a lot more, I bought a new-ish Treo 650 on eBay. All went well until I tried to send SMSes using my Optus SIM. No luck - I got an error 4027 every time I tried to send a SMS, although voice and GPRS worked fine.
First attempt at a solution was to set the Message Centre Number in the phone. Strangely, the Message Centre number (configured though the SMS app) was always empty, and when I tried to set it to the correct number for Optus, the value wouldn't stick past a phone reset.
Thanks to Google, I found the outline of a solution on a message board.
In summary, the Treo 650 (unlike the Treo 600) is not compatible with Optus's standard post-paid/plan SIMs used in Australia. However, it is compatible with the Optus pre-paid SIMs. If you have a Treo 650, and a post-paid/plan SIM, you can buy yourself a new pre-paid SIM for $4 from your local Optus store, then ring the Optus call centre to have them do a "SIM replacement" to move your account and number to the new SIM. This solves the problem within minutes of the "SIM replacement".