Anybody, anywhere in the world, can now rent inbound phone numbers and make outgoing calls using SIP trunking providers. This is often the fastest, cheapest and most flexible means of getting connected to the PSTN. It is often possible to port existing phone numbers from legacy phone companies to SIP operators.

There are a range of hardware options for joining analog and ISDN telephone lines to an IP network. One option is to purchase a dedicated media gateway, such as the those promoted by Cisco Systems and other well known vendors. Another option is to install an ISDN or analog telephony card into a server and run Asterisk or FreeSWITCH, as described in Chapter 17, PBX Setup, configured solely to act as a media gateway.

If dedicated ISDN links to the telephone company are important, it is worthwhile considering one further permutation: instead of installing the media gateway at the office location, have the ISDN lines installed to a rack in a data center and use VPN or WAN links from the data center to the office. This can achieve higher resilience and flexibility: if the main office site suffers from any kind of technical or environmental disturbance, calls can be routed from the data center to users at home, on mobile devices or at another branch office or disaster recovery location.