In the case of Spain, the Government established in the BOE of August 2005 that the prefix for VoIP nomadic numbers would be 51. So, the Secretary of State for Telecommunications and for the Information Society, has defined nomadic numbers as:
"Publicly available electronic communications services that offer two-way voice communications in real time from access points to which users can connect remotely and allow both the establishment and reception of calls, and may additionally include other types of capabilities such as multimedia communication."
These numbers were created to provide companies with a telephony service throughout the national territory, regardless of the geographical location in which the company is located. The only requirement is that the subscribing company resides in Spain.
Traditionally, many companies used the commercial prefixes 902 / 901. However, the advantages are obtained when using the 51 prefix, because at the time of receiving the invoicing it should appear as a landline. And, therefore, the cost of the calls made would be included in a flat rate, although there is always the exception to some companies that bill per minute.