To Connect Audio Generator to Phone Call?
As you've already said you don't mind typing on your laptop to achieve this, it's perfectly possible. Just use any text-to-speech system on your laptop (Windows has one built-in, and you can buy/download extra voices for it), and make it output to your laptop's headphone jack. Then you need an adapter cable to connect the headphone output of the laptop to the handsfree kit of your phone. Alternatively, you can use your PC as a Bluetooth headset for your phone instead of using a tangible cable.

Note that this technique isn't specific to Android at all: it would work just the same with an iPhone or even a 'dumbphone'.



Doing it on the device might not be possible, even with a custom ROM. My answer here about recording calls explains why not all phones can do this: often, the phone call audio goes directly from the Bluetooth interface or speaker/mic to the mobile baseband, so the application processor has no way to send other audio to it (or to intercept the received audio).