I certainly wouldn't do it, but you could use e-mail to change the password. Use procmail to trigger a script that extracts the users name and the new password from the e-mail message then updates the password file.
A better solution would be to have the modem answer incoming calls. Login and "Bob's your uncle". To increase security you can have cron only allow logins during a specific time period or setup call blocking on the line to only allow calls from certain numbers.