I finally succeeded in finding a phone that works with my computer: Motorola PEBL U6.
I sent a few SMSs and they all went out successfully. However, I noticed that my mobile subscriber deducts money for each SMS sent. I thought sending SMS via FLSMS is Free? Please correct me. Can it be free to send SMS via FLSMS?
The FrontlineSMS software is free, but you have to pay for messages you send, and you do this in the normal way. Details of this are on the website. Hopefully you'll still be able to use the software now that you've got it running.