I would suggest you make your voicemail message something to that effect. That way, if you don't pick up, your caller will know you will get back right away (or as soon as possible).
I know a person who NEVER picks up and his voicemail says "Don't leave a message; I'll call you back as soon as possible." He gets the number/ID from his missed call list and doesn't waste time on voicemails. Needless to say, he is extremely busy in his business life and this works for him.