Typically ISP’s only provide clocking when connecting to them (ie MPLS). If you have point-to-point T1’s, you usually need to provide the clocking. On one side you can leave the clocking to the default of line. On the other side set the clocking to internal.
controller T1 0/2/0
clock source internal
channel-group 0 timeslots 1-24
If you have multiple T1’s, some private and some to a carrier, you may need to set the lines to the carrier to not use the internal clocking. To allow the ports on a specified network module or voice/WAN interface card (VWIC) to use the network clock for timing, use the network-clock-participate command in global configuration mode. To restrict the device to use only its own clock signals, use the no form of this command. You cannot have a multi-port card participate in two clocking formats.
no network-clock-participate wic 0
Here is an example of mis-configured timing. Latency was high and packet loss was consistent at about 2%.