Asterisk as transcodec proxy between phone and softswitch

Hi, i have a safari softswitch that doesnt support g711u, only g711a, asterisk supports both, how can i configure asterisk to make the transcodec between the softswitch and the sip phone (that only supports g711u), i mean something like a codec proxy (im not sure how to call such a beast…), the sipphone will register with the softswitch, so iin asterisk there will not be those extensions.
i hope i made my question clear enough