We want some sound proof phone booths for our office.
Use plexiglass.
The design would be dependent upon the distance from other sound sources such as bathrooms, roads, airports and manufacturing processes. The average dB level A rated of speech is 60 dB, toilets are 75 dB A rated. Manufacturing and other sound sources depend upon distance from the sound source and the existing structures ability to absorb sound. Additionally phone lines have a restricted narrow frequency range of 300 to 3300 Hz, called the voice-band, where sound absorption emphasis should be placed.