You are essentially referring to a "world phone." Some phones like say, some Blackberry models operate on CDMA and also have a SIM card slot for GSM. My phone, a Blackberry Tour runs on CDMA usually, as that is what my provider has. However, it also has a SIM card slot if I were to travel overseas or switch providers. Essentially, you are locked in to whatever technology you are using at the time. If you have the SIM card in, the phone will only operate using GSM- not CDMA and vice versa. So really, there is no need for a call manager, etc...
Actually that's not correct, you can have a SIM card in, and then swap networks.
I have many CDMA + GSM Roaminng phones, and all of them let me chose the network when outside of North America (like in China I can pick CDMA or GSM partner networks).
Your Tour should also allow you to do the same thing (unless locked out by your provider/carrier). Go to Manage Connections, and then Mobile Network Options, and then Toggle your Network Selection mode if necessary and then chose Network Technology to switch from 1X/EVDO to GSM/UMTS or the Global option.
For most phones the network manager is in the options, but it's not like a quick welcome screen toggle it's usually buried in the options menu.