You shouldn't be worried if there's not other signs or past problems of her cheating on you. Many women and men can't live without their cell phone and keeping in touch with their friends. If there are not other signs such as leaving the house alone more lately than before, not wanting to spend time with you or being vague about where she goes or does than I wouldn't worry too much.
I don't think secrets are good in any marriage. If she is just really into using her phone then let her be. My husband is always texting friend, it's his way of keeping in touch I guess. I would casually ask her who she is chatting with then engage in the conversation, ask how they are doing, job, etc...make it sound like you are just being curious and keep the questioning innocent, don't be accusing or nagging. If she is just keeping in the loop with some friends then she shouldn't be bothered by your questions. If she becomes defensive, or aggressive then there may be something more to worry about, but don't jump to conclusions just express how you feel to her. Good luck:)
From personal experience I say yes... Only because it happened to me and come to find out she was communicating with various guys, then found out she did cheat. Look for other signs before you say anything. It's possible she's texting someone and is scared to put the phone down for fear you'll hear or see the incoming text. Hope it works out for you.