Touma calling Mikoto
I'm just at Index III and was wondering how mikoto could send Touma a message, then I remembered that he once before called her for Index to ask questions about Science in the Academy City Invasion Arc (episode 21 - Index II). Just wondered how he got her number and when that happened, I figured that it may have happened in the episode "Penalty Game" when she goes with to a cellphone service store to make a contract as a "couple" and to aquire a gekota strap. But it wasnt ever explicitely stated. And it must have happened somewhere in Index II because of how they meet in the first episode where she was trying to ask him to her his phone number.
So can anynone help me find out when this happened? Or maybe it was only stated in the Light Novel?