Asumming it's a 7940 or 7960, make sure telnet_level in your tftp config file is set to "1" or "2", not zero. Unfortunately you need a TFTP server, this isn't something you can set through the phone interface.
(Create a file called "SIPDefault.cnf" , or "SIP" mac_address_of_phone ".cnf", and have "telnet_level: 2" in it. Better yet, download a SIPDefault.cnf from Cisco's Software Download and modify it to suit.)
If it's a 7905 or 7912, there's no telnet interface, but an HTTP interface instead.