I have a Telstra mobile account and don't know how I can make a phone call in Souillac, France.
If you mean that you are in Souillac and need to make a local call, then you need to have international roaming activated on your mobile account. Telstra do not have this turned on by default, you have to ask them.
Just call them up on +61 439 125 109 and they will activate it for you. Oh, wait....:)