SIP Trunks are emerging as the de-facto replacement for PSTN connections. Traditionally, these have been fixed wired into buildings, provided by a local telco and in the form of COT (analogue) or BRI/PRI ISDN (digital) telephone lines.
ISDN in many countries is timetabled to be discontinued in the next decade, and it's often argued that SIP trunks have finally come of age as an alternative which has more advantages than ever over ISDN, whilst maintaining, or improving on the quality of, the service and reliability.
SIP Trunks can be delivered over any kind of data connection, from ADSL, all the way up to fixed 2, 10, 100 or 1000 Mpbs connections. The main aspects to consider are that the bandwidth has to provide a constant 100 Kbps per call, and the latency in one direction should not exceed 100 milliseconds. Some data connections will also provide a level of "Quality Of Service" (QoS) to guarantee the speech is delivered 100%, all the time, and other links may not. It's important to consider this when comparing to the traditional types of PSTN connections.
It's also important to consider that Analogue and ISDN lines are relatively difficult to hack. They require a degree of physical interference with the lines between your property and the local exchange, along with a reasonable level of expertise. On the other hand, if SIP traffic passes over public internet spaces, you can consider it 'open season' for the world to try and molest it. You also have to consider that when SIP is delivered over internet connections, you're opening up doors to the world, which never needed to be open - and they will be scrutinised on a daily basis.
Having said all that - a well managed implementation of SIP trunks can save you money, can improve the quality of your phonecalls and can be secured so as to cause no concerns.