In theory, any SIP ATA or softphone can talk directly to any other ATA or softphone without an intermediate provider. In practice, this is often very difficult. In order to call another device you must know the user name, IP address or FQDN, and SIP port of the destination device. If you wish to call from a telephone connected to your ATA all of these parameters must be numeric. Additionally, the receiving device must have ports mapped inbound through any NAT router or the cal will not be successful. For all these reasons it is often simpler to register each device on a service provider like FWD and rely upon them for all of these services.