SIP-Trunks are the newest kind of dialtone
SIP-Trunks have the capabilities of traditional PRI phone service without requiring a pricy dedicated T1 circuit. Often SIP-Trunks will have allow the vendor/user to redirect individual numbers for failover to a pre-determined alternate number (eg a cell phone). Thus if the service is interupted by a circuit down or equipment issues the service can re-route calls to alternate number(s). SIP-Trunks generally ride over your internet circuit. SIP-Trunk service is often the most economical means of getting phone service today.
Port your existing phone numbers IN and keep all of your old numbers. If should leave NEC you may take your phone numbers with you. NEC SIP-Trunks are currently only for NEC systems. We are working on an exception for our Toshiba customers.