Or use a shell that supports backgrounding processes (pretty much anything more advanced than sh will do this). Hit ctrl Z then type bg to allow the telnet to continue in the background.

When you are ready to go back to it, type fg.

If you have a few things running and need to pick which one to foreground, type 'jobs' to see all running process associated with this shell, then fg %2 to jump to job number 2 for example.