I am generalising here, but they tend not to use software dialler. They tend to use normal telephones connected to VoIP gateways. The gateways send the calls to an Internet Telephony Service Provider over the Internet.
Cisco ATA 186s are popular. They cost about USD 140 each. They are two port gateways. That means, you can connect two telephones to them and make two calls at the same time. The Elesign 4800 is a 4 port gateway and the Quitum a800 is an 8 port gateway.
All these units are compatible with our own calling service, DUAL Talk.
Please let me know if you need any further information.