iptraf is run from the command line on your Asterisk box. Go to the LAN Station Monitor section, and look at the InRate and OutRate for the MAC address of the remote Asterisk system. Note that I can’t see a way in iptraf to see the rate to/from an IP address, just a MAC address.
Alternatively, iftop will give you a rough idea of the bandwidth in use between various hosts. Pressing ‘L’ for a logarithmic display may help you.
Note that SSHing into your Asterisk box consumes network traffic.