Has anyone managed to make this combination work?
The E173 works in the PC but does not appear to be noticed when in the router. I expected the online status to show a 3g modem even if it was not connected, but no sign of any change to show the modem is there. I have set the backup to always on, and configured as per t-mobile settings.
Any suggestions?

The E173 is much too new for the 2800 to have any idea how to interface with it - I recommend getting hold of a Huawei E156, E160 or E169 modem if you're gonna use 3G on the 2800 series.

Thanks for the quick response. I was hoping that there would be a special initialization string I could sent to kick it into life, but I did have the feeling that I was wasting my time. I have now found the release notes for the 2800 firmware, which mentions a number of supported devices, including the ones you suggested. The Huawei E220 was also listed, so I may give one of those a try as there are a few around on Ebay at the moment.
Regards,

I now (second attempt) have an E160 that works on my PC, but no luck yet on the router. The led flashes to indicate that it has found a network, but I cannot see anything on the router that tells me that the E160 has been seen.
Do I need to tweak the modem strings from the defaults?