There is a special 'cellphone mute' input wire on the RioCar docking sled wiring harness.

Read your RioCar manual to identify the correct wire (and its colour) and check that the signal output from the Onstar interface is either +12V when external circuit is 'on' and 0V when its not,
or
its 0v when its on, and +12V when its not.

Assuming the voltage is no more than +12V [and if its less it may still work - but you'd have to try it to find out],
then connect your Onstar signal wire to the Cellphone Mute input and with some luck, when the button codes screen in Hijack is active you should be able to see the 'fake' button code being generated by the kernel whenever your onstar system is in use and when its not.

Once you have the codes, then its a simple IR map to do whatever it is thats required
[pause the player, switch to Aux input, unpause the Aux input source etc].

[and when the phone is finished with, switch to player, and unpause].

Hope this helps.