Setting off proxying of voice

When I call from one softphone to another via the Asterisk using the same vocoder (gsm, for example) the trasnalator gsm-slin-gsm is creating. But the vocoders are the same - I think the voice transmission should be straight, without decoding&encoding. How can I manage to do this?
This all only about the H.323. Calling via the SIP doesn’t cause the translators creation.