First of all, it is important to distinguish between PSTN and ISDN.
PSTN stands for Public Switched Telephone Network and is sometimes called Plain Old Telephone Service (POTS). It is a traditional analog-based infrastructure for making and receiving calls over circuit-switched copper wires. The wire/network is owned and operated by your provider.
ISDN stands for Integrated Services Digital Network. The network allows voice and data services, such as video conferencing and data transmission, via digital lines. ISDN digital services can provide multiple channels on each line so that phone calls can be made at the same time.
ISDN is more modern than PSTN, but both are now considered outdated and inferior to PSTN, and are strengthened by the fact that both will be closed in the next seven years or so.
What are the options?
The most popular and common alternative to traditional PSTN and ISDN is Voice over Internet Protocol (VoIP), which transmits voice traffic by converting Internet voice traffic into data "packets". Many companies are already using VoIP solutions.
What should you do now?
If you are currently using a PSTN/ISDN system and your contract will be renewed in the next few years, it is definitely worth considering switching to VoIP. The many benefits of this include cost savings, increased mobility, video conferencing and easy expansion
