Which pda? if Palm the best place to start is www.palm.com there is a free environment that you can use to develop code for these devices. If you want to develop code for WINCE (yes its pronounced that way) you actually can use VB but there are limitations and the development system will cost...
Need far more info than you have presented. How are you entering this string into what type of variable from what form or is it a console app? Are you converting from a string to a number or is it a number field that you are entering the data into? Please post at least a code snippet to give...
I'd just use winsock to connect to the port. send text and capture results. Telnet is not a particularly difficult thing it's really just a dumb terminal.
