My voicemail number (+1 914 449 1795) is intended to provide a contact number for my domain registries. The reason for this is I used to have my telephone number as the contact for domain registries, but I would get constant calls from scam artists, or unethical companies trying to sell me web design services for my “business”. I eventually got tired of this, and set up a voicemail only number. Once that propagated through, I eliminated all of such calls. In addition, such companies choose to not leave a voicemail message which is fine by me. 95% of the calls to my cellular phone is legitimate.
A Domain registry is expected for anyone that owns their own domain (such as fsp.ws). With that in mind, they require a telephone number, email address, and a mailing address. This information is required, and while some companies offer private WHOIS records, this extra cost service is only valuable if all domains supported this. The voicemail number is $1/month, and therefore not a significant cost to me.