I really love Highrise. It is a 37signals service that helps you organise your contacts, your conversations with these contacts, cases, deals and tasks. Highrise also sends task notifications using SMS. Unfortunately it only supports specific providers for certain countries only. My country (Greece) as well as many other countries all over the world are not included in that list. Nevertheless, I managed to make Highrise send notifications to my mobile phone through Google Calendar.
Step 1: Find out your Highrise iCalendar URL
You do that by navigating to Tasks and looking at the bottom of the page for a link that says “Subscribe to your iCalendar task feed”. Clicking on that link will take you to another page where you can find your iCalendar URL. You copy that and you go to the next step.
Step 2: Go to your Google Calendar
Locate your “Other calendars” panel and click Add, to add a new calendar. Choose “Add by URL” in order to enter the iCalendar URL you found in the previous step.

Enter the iCalendar URL in the “Public Calendar Access” field and press “Add”. That’s it. This calendar will automatically contain all your Highrise tasks. You can go to the calendar settings and add a default notification setting for SMS. That way, every Highrise task will end up in your mobile phone.