My N900 telephony features have been going in and out for the last week, until I completly lost all phone function.

Figured it was my software so i reflashed it to pr 1.3, but didn't reflash the file vanilla emmc cause well it just would not remain in usb mode when i tried running flasher 3.5. Flasher 3.5 sees it in usb flash mode but then it doesn't and the usb symbol in the upper right hand corner disappears and flasher cant see it again.

The thing seems to work anyway. It seems to boot up just like a new n900, yet im still having the no sim problem. Swapped the sim card for my brother's sim. We both have tmobile. Still no dice.

Please look at this thread http://talk.maemo.org/showthread.php?t=53335 as it explains everything you need to know

You also need to flash the latest vanilla emmc.

i got these kind of problem long time ago but i use the old method way rather then reflash my n900...

i take out my sim card n bend it a little then walllahhhhh problem solve....
u can push down the sim card holder on your n900 aswell...works the same without having to flash your phone....