Listen/register 2G 3G change on android

Question

I know how know network type with :

TelephonyManager telManager = (TelephonyManager) context.getSystemService(Context.TELEPHONY_SERVICE);
int network_type = telManager.getNetworkType();

But I want to register when the phone switch into 2G to 3G or 3G to 2G...

Anyway to do this ?

Thanks for helping me. Regards Jim