If no DSPs are available, you need to configure dspfarm or install a PVDM (Packet Voice DSP Module).
Router(config)# card type t1 0 0 Router(config)# controller t1 0/0 Router(config-controller)# clock source internal Router(config-controller)# framing esf Router(config-controller)# linecode b8zs Router(config-controller)# no shutdown port1 no voice service configured
If you are using SIP registration (e.g., connecting to a VoIP provider like Asterisk or Cisco CallManager), you need a voice register pool: If no DSPs are available, you need to