iptables & NAT & IPSEC can ping but cannot telnet ?

Hi
I had a linux box[A](with NAT but no IPSEC) setup ppp dial to ISP.
And another linux box[B] used IPSEC behind box[A].
I forward any input package from box[A] to box[B](192.168.1.10).

# echo "1" > /proc/sys/net/ipv4/ip_forword
# iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
# iptables -t nat -A PREROUTING -p all -i ppp0 -j DNAT \
--to-destination 192.168.1.10

When box[A] got an IP for example (170.8.23.33) from ISP.
I ping box[A] from Internet with other linux box[C]
it also with IPSEC it's OK.
But when I try to telnet to box[A] from box[C] the telnet
show "Connection timed out" messages.
Why ??
How can I do to make the telnet work.
Thanks.