SippySoft works in a pass-through mode, so codecs received from client's UA are passed through the system to the vendor without transcoding.
For instance, the call can't be connected in G.729 codec if caller's UA doesn't support it even though vendor requires it.
On the other hand, SippySoft have a trans-coding feature added to the product roadmap (request ID: SS-693).
Nevertheless, there is a possibility to define a "Preferred Codec" option and enable "Use Pref. Codec Only" check-box in account's settings.
"Preferred Codec" option - change the priority order for the codec list that Sippy received from the remote side in the incoming request(see screenshot for your reference).
"Use Pref. Codec Only" option - excludes all received codecs from the SDP and forwards only the preferred one. Thus all calls under this account will be handled only in preferred codec).
In case the caller is not sending preferred codec, Sippy will cancel such call without routing to vendor.
Those options may be found in the Account's properties: