Yep, that can work but not with GSM as you use thirdparty hardware. So this is how I resolved it. We had the same issue in the company, the result is that we use the estate where the office is as test bed.
Yeah yeah "free" internet and VoIP phone for the whole estate I know I know .
Anyway we use WiFi, reason being is that we see that more and more devices use WiFi. We have HP6515 with Wifi cards. So GSM calls are routed via GSM, however when we are in our Bee Hive (coverage and overlapping coverage by WiFi using the available WiFi devices in the estate) we use the PDA's to call via VoIP.
Now the trick is you can expand on this, everywhere you have a WiFi hot spot you can use your VoIP (taking VPN etc in account) You need some SIP software on the PDA but that is available on the internet.
D-Link now comes out with WiFi phones and for sure there are more suppliers that will come up with WiFi GSM phones.