If you are checking emails while you’re out and are responding slower than normal, state that, suggests Jill Gugino Panté, director of the Lerner Career Services Center at the University of Delaware. “If you don’t have access to email and can’t return messages, state that as well,” she says. “The clearer you are in your messaging, the better.”
I had a colleague who simply never answered her phone or set up her voicemail. She still listed the number on her business card and email .sig, she just never answered or checked voicemail. When she took a new job another colleague inherited her phone number and when he went to set up his voicemail there were basically eight years’ worth of messages left for her that he had to delete.

One of the only reasons I get voicemails is because our system is set up to send new voicemail messages to your email as an attached file. Now if only it would send the voicemail as a transcript, I’d be set. I don’t mind returning calls, but listening to voicemails is obnoxious, especially because people are really bad at leaving voice messages.
Also, IMO, it’s putting the burden of knowing and managing your schedule on the other person. You expect them to be paying enough attention to you to remember when you come back and to resend their request. That’s not reasonable.