Out of office messages are important because they let people know you are away from your regular work duties. These messages help businesses and projects run smoothly when a member of their team is away. They let people know you received their message and will respond by a certain date. This helps prevent confusion and frustration if someone is trying to communicate with you by email but not getting an answer.
I’d just stick with “I will be OOO without access to phone or email from XX/YY to XX/YY, returning on XX/YY.” And then whatever directions for directing to your support/backup while you are out. I find that specifically saying ‘without access to phone or email’ sets a good expectation of non-response.

I might sound nitpicky but the language is important. “Might” or “may be” or “slower than usual” are vague and don’t offer the sender all that much information about when you’re really going to respond to them. Worse, they do a horrible job of protecting the time of the email receiver who, as the responder notes, is not in the office! Such a responder implies that, not only will the vacationer reply to the email, but they may not even miss a beat. They may be slow to respond, but they also might not.
Such a system sends automatic replies to text messages and calls that you miss when not available in the office. A fully-functional app like Calls and SMS Auto Reply gives you a simple, convenient way to respond automatically to people trying to contact you when you are out of the office.