You can change the port number for telnet using simple IP NAT. Create a loopback on the device you want to change the port on and use this as the NAT inside. Use your LAN interface as the NAT outside and use TCP port translation. e.g R1 telnets to R2 using outside global address 99.99.99.99 port 99
R2
----
Interface Loop 0
ip address 2.2.2.2 255.255.255.255
ip nat inside
Interface fast 0/0 ;local LAN
ip address 192.168.10.10 255.255.255.0
ip nat outside
ip nat inside source static tcp 2.2.2.2 23 99.99.99.99 99
ensure R2 has a route to R1 and ensure R1 has a route to the inside global address 99.99.99.99 via R2.
From R1 you will be able to telnet via port 99 e.g.
telnet 99.99.99.99 99
Tie down all tcp port 23 traffic via access-lists. Almost a basic IOS firewall in reverse and works with most IOS.
Hope this helps.
Johnathan.
CCIE#9332
