We use Serverscheck
www.serverscheck.co.uk and clickatell for our monitoring/sms system. It's not the most intuitive software around but it was easy to set up, not too expensive and it's worked well so far. We run it on a windows box in our office (adsl connection).
Note if you use clickatell make sure you're on the UK roaming profile, or if anyone has ported their number between mobile telephone providers (such as myself) they won't get the messages. Clickatell customer services will sort this out for you.
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