the fax. This allows many recipients to have an individual fax number, even though there is only one fax machine available. Some voice over IP (VoIP) vendors have used one central, remotely located fax server as a means of offering Internet fax service to their clients. In theory, standards such as T.38 should have allowed VoIP subscribers to keep their existing fax equipment working locally; in practice, T.38 at the subscriber's site offers no benefit if the upstream provider is least-cost routing to gateways that do not support T.38 and cannot reliably send or receive fax/modem traffic. A fax server at a central location, connected directly to public switched telephone network (PSTN) T-carrier primary rate interface lines and using direct inward dial to identify the intended addressee can convert an incoming fax to an electronic document (such as .tif or .pdf) for web or e-mail delivery. The fax traffic never passes through the VoIP infrastructure as a dial-up modem call and therefore arrives reliably even if T.38 is not properly supported at some points in the network. Voice over IP DID service has similar relevance for Voice over Internet Protocol (VoIP) communications. To allow public switched telephone network (PSTN) users to directly reach users with VoIP phones, DID numbers are assigned to a communications gateway. The gateway connects the PSTN to the VoIP network, routing and translating calls between the two networks. Sellers In countries with multiple competing local providers, DID services can be purchased in bulk from a Competitive Local Exchange Carrier (CLEC). For voice over IP resellers, some specialized CLECs (for local numbers) or interexchange carriers (for toll-free numbers) will deliver blocks of direct inward dial calls already converted to Session Initiation Protocol (SIP) or common VoIP formats. The individual VoIP provider need only obtain an inventory of local or freephone numbers from VoIP-aware carriers in various regions, import them in bulk to an IP PBX and issue them individually to end users. International DID numbers can be purchased in bulk from international providers. UK geographic DID numbers can often be obtained for free and can be terminated over SIP. A few US DIDs are available without monthly charges from vendors like IPKall (discontinued in 2016), but at the expense of the caller paying for a call to some expensive, rural location. The majority of vendors charge a nominal amount per number per month (as little as $1/month