There are no special patches or something.
i flashed it completely, updatet it, installed the CSSU-Stable.
Rootsh, FasterApplicationManager.

I had the same problem with my old Nokia N900, kept killing a call after a certain length of time, Got a new sim card and didn't help the issue. Also cut out the Mobile Data connection after a certain length of time, kept showing the sim card not inserted icon then cut off. I returned it to the store and got a new one and had no problem since. I'm assuming it was a hardware/software issue. I was using o2 uk.

It could be temperature dependent. As regtheledge noted, if the SIM shifts from internal heat and drops contact it could kill the call. In your case, I'd doubt it's that though, since it would make immediate follow-up calls do the same thing. It being so precise, I'd be willing to bet you have something happening periodically that's causing it.

So what happens in 10 minute cycles? Wifi refresh? Do you have a position app installed (like Zaploc?) that's turning on GPS periodically? Next time before making a long call disable GPS, GPRS and Wifi and see if it still happens. (There are scripts and apps that disable wifi without disabling calling.) If that fixes it, turn them on one at a time and find out who the culprit is. That at least may help narrow your search.

I have a similar problem.
I can't do phone calls longer than 10 minutes. Phone drop the call. It shows the icon that SIM card is missing for a short time. Call drops in various tiems. Iimmaediately or after 1-2 mins or 5-8 mins. Strange but if I call the operator's customer support number for testing - because that number is free - usually call drops after 9:59 exactly. While call to other numbers call drops after various times but never reach 9:59. Even in 2G or 3G. Indoor or outdoor. I tried three SIM cards from various providers. Tried reflash the phone but doesn't helped.
I call the operator, what if the phone is stolen phone (my phone is a second hand unit) and the owner disable it by the IMEI number. But operator said, in this case phone can't login to network.
Official Nokia servie can't help me. They ask for the IMEI then the pepole said this unit is from the USA and they can't repair. Then I go to the 'GSM shop' now I waiting.

I tested it yesterday with another sim, same Problem.
i don't think its the heat, because after a kill, you can immediately start the next call and it lasts 10 minutes.

i'm going to try it with GPS/GPRS/... off
tomorrow i can write about my experiences.

I could nearly guarantee it was not an operator issue as I had the exact same problem and tried all different thing including having no GPS or Data connections active & Switching between 2g & 3g. The fault was definetly with the device. I bought mine's second hand also but had a full clean flash before i purchased so could nearly put it as a bug/hardware issue.

just reflash it again

tried this first

I've tried yesterday all different connections, 2G, 3G, tested also mobile connection. still having the bug. Only wifi works fine. on wifi there are no disconnects.

I just wrote a supportmail to Nokia, let's have a look what they say.

I dont want to sent the device to Nokia, last time i did this, they gave me a C7 -.- (My device was working, but usb broke, they don't sent me back my N900 )

@StickYY,

Good luck!
Please do let us know if/what Nokia replies!

BTW did you just say that this not only affects calls but also GPRS/UMTS connections?

@reinob it affects UMTS/GPRS and calls. Only Wifi is working fine.

Nokia called me back, i should sent the device to Nokia, but if the problem is on the hardware, (they have no more spare parts) they will sent me back a N8 or E7(their informative: no there is no way to get the broken N900 back). If the problem is on software they will fix it.