[Boromir88]

I don't even think Saruman could have contacted Sauron, he got stuck in a no win situation. The entire time Saruman was looking out for himself and planning to betray Sauron. Sauron eventually figured this out, but the point is, as Gandalf tells Theoden et all after Pippin looked in the palantir, Saruman got his caught in a vice, and his charade was up.

The "good" side knew Saruman was a liar, Sauron became aware of his intentions, and we see when Pippin uses the palantir Sauron assumes he is Saruman's servant and asks Pippin why his master had neglected to report in a while. Saruman had no escape, Isengard was destroyed, his power in the Council and in the Order of the Istari was gone, he was trapped in his tower, with no power to hand Merry and Pippin into Sauron whether he told Sauron or not.

Gandalf speculates that Saruman does have the power, while in Orthanc, to maybe trap one of the Nazgul, but other than that he's got nothing. Saruman's choices were be imprisoned in Orthanc and take a chance with the good guys who would spare his life, or let Sauron know that he was virtually powerless. Isengard was lost, even if he did tell Sauron he had two hobbits, Saruman was imprisoned in a tower, he couldn't hand them over to Sauron, and facing Sauron (even when he did have his power!) was the last thing Saruman wanted. Saruman was obviously bitter and spat right back in Gandalf's face, but they weren't going to kill him, it was better to be imprisoned by Gandalf et the "softies," then to let Sauron know he was powerless and had nothing to offer.