An organization that provides voice over IP (VoIP) service and enables calling from SIP phones to plain-old-telephone (POTS) lines and vice versa. SIP is the universal protocol used for Internet telephony, and there are more than a hundred SIP providers worldwide that provide the POTS/SIP conversion.Inbound - SIP Origination/SIP TrunkingSIP origination allows a POTS telephone number to be dialed and answered by a SIP phone. SIP providers have thousands of local phone numbers in local area codes that can be assigned to their subscribers, and subscribers can allow their customers to phone them toll free from different parts of the country. Family and friends of subscribers can also take advantage of the local phone number. In addition, when switching service from a landline to a VoIP line, the landline number can often be ported to the SIP line.Outbound - SIP TerminationSIP termination lets subscribers call cellphones and landlines from their SIP phones. The rates vary depending on country and provider. See SIP, SIP trunking and VoIP.