This was actually an arcade standard specch synth, using the General Instruments SP0256b Speech chip as found in arcade games like Astro Blaster and Star Trek. Obviously it needed a dedicated cart to deliver speech - games like B17 Bomber and Bomb Squad which I haven't got so I don't know what this little box sounds like.

As explained in the pix below the cover on the top of the intellivoice (with 'mattel electronics' stamped on it) was designed to be removeable and underneath there's space and solder pads for another cart slot....what this was planned for I've got no idea!