Why is Touya hiding from Yuki that Rina called him? They didn't do anything wrong. It makes him look guiltier if he hides it.
I'll try to answer this as a whole since no episode or season has been specified.
Touya feels bad throughout the entire series and knows he's not really up to much good. In fact, he pushes away the girls who has committed some form of "adultery" with away many times. For this reason, we can conclude there is at least some feeling of guilt brooded within him.
However, the most plausible explanation (note, we can only speculate as the series never says) is Touya has feelings for Rina and does not want Yuki to know they talk outside of business. Touya spends a lot of time with Rina on the job a few times during the season but it was supposed to be strictly that.
One can only assume out of guilt and uneasiness, the natural reaction was to hide personal relations with Rina so that Yuki would not think less of him.