I have a analog phone conected to a fxs port in the 2821 router(using sccp). If this analog phone call to a ip phone (local or remote), is transconding resources requiered ?.
The FXS port is going to use DSPs no matter what, since it is an analog connection. Since these are the same DSPs that do transcoding, it's fairly equivalent to transcoding.
Depending on what codec CUCM negotiates with the gateway, the DSP will simply change what codec it is using.
This isn't something to worry about, unless your DSP resources weren't allocated correctly for high complexity codecs.
You can use the DSP calculator to be sure: