TravelSim comes with either an Estonian mobile number OR an Australian mobile number. You'll need to ask your friend what is their TravelSim number (sorry, we don't know).
If it starts with +372 then it is an Estonian mobile number and it will cost 96c/min to call and 25c/text sent. You'll need to add extra credit with your purchase to call or text.
If it is an Australian mobile number then your plan already includes unlimited texts to Australian mobile numbers, but you will need to add the International Mobile Calling Pack to call them.