I am using Iphone 4S since 5th Jan 2012 ago & I got the same problem with u even I updated my firmware to IOS 5.0.1.
I can't end my call and it keeps on hanging at "call ending" status & can't go thru into "call ended" status. Especially quite frequent happen when I switched on my Iphone in the morning & made my first call!! Hence, I can't make the phone call after this.
When the above issue happened, I tried to use other mobile phone to call & sms my Iphone & It is ringing but my Iphone does not detect any incoming call & also not receive sms even the sms is successfully delivered. It is randomly happened.
But I don't notice any "no service" with my iphone signal. My reception is ok all the while.
I suspect that both the IOS 5 & even updated IOS 5.0.1 have bugs because u mentioned that before u updated to latest version (i think should be IOS 5.0.1, right?!) your iphone 4 was working perfectly well. Somemore, I found out from other Communities Discussion that the IOS 5.1 beta 3 has fixed their iphone 4s "outgoing call no audio" issue so far. Thus, we should hope that coming IOS 5.1 will fix this ridiculous issue as soon as possible.
I was also advised by the Apple Technician that try "reset all setting" or do the firmware restoration to determine whether is software issue or not. If still happen, then try to replace a new micro sim card. If still happen again, then we should surrender & it seems to be the hardware issue & should claim for hardware warranty to replace a new Iphone 4s.
Anyway, I suggest we should wait for IOS 5.1 officially lauching to check whether can solve our issue or not because we are not using the same model of Iphone but facing same prob with IOS 5.0.1 & by right, your Iphone 4 should not have hardware or the micro sim card issues so far.
Did u use bluetooth headset & what bluetooth version u are using? Because at first, I suspect my old bluetooth headsets with version 1.0 & 2.0 (either bluetooth software / hardware) cause these problems. So I bought a new Jabra with Bluetooth version 2.1, but still happen.
Hence, I suspect if this is hardware issue then maybe due to signal / reception hardware device problem or bluetooth hardware device or wifi hardware device problems. Or maybe also bugs from particular downloaded apps!!
Anyway, my suggestion to u is u try to uninstall your itunes with ur IOS 5.0.1 completely & download again the itunes & IOS 5 without enable any virus scanner, windows firewall & defender. After downloaded, u should use iphone recovery mode & do the firmware restoration. Of course u need to backup before proceed with firmware restoration.
Is better if u got another laptop & just download the new itunes & IOS 5 & remember to disable ur virus scanner, windows firewall & defender in the process of download & install them.
hope I can help u a little bit & we hope together that Apple Team can identify the real problem & solve these issues as soon as possible & don't just let us try this & that method & wasting our golden time as we spent a lot of money on Iphone is not let us sufferred but should let us enjoy using it completely!!