In the last two chapters, we have prepared OpenSER to handle calls using authentication and a database. SerMyAdmin was used to handle the database records. However, you still can't send calls to ordinary phones, because you are not connected to the PSTN. The challenge now is to route calls from and to the PSTN (Public Switched Telephone Network).
To send calls to the PSTN, you will need a device called a SIP PSTN Gateway. There are several manufacturers of this kind of equipment in the market such as Cisco, AudioCodes, Nortel, Quintum, and others. You can also use an Asterisk PBX box for this task. Asterisk makes an affordable PSTN gateway that is very competitive with the big players mentioned above. It is fully open source, licensed according to GPL.
By the end of this chapter, you will be able to:
Interconnect OpenSER to a SIP gateway
Apply permissions to inbound calls
Use ACLs to protect the PSTN gateway from unauthorized use
Use the LCR (Least Cost Route...