I haven't used iptables in a while, but I believe you have to speicfy a protocol option. So it would look something like this:
iptables -A INPUT -p tcp
--destination-port telnet -i ppp0 -j ACCEPT
You might want to check this link out, it doesn't really go in depth but it clarifies a lot of things, at least for me it does. Click Here!
EDIT: This won't actually "open" a port, but rather allow connections to the port you specify. You first have to enable the service you want people to connect to.