RTP Payload Size

I want to connect Softphone with asterisk using DialUp Line. i.e asterisk is also connected to modem and softphone too. I can successfully login to asterisk server using dialup line but voice quality is too bad. How can I reduce RTP Payload Size or is there any software base solution for Header Compression

If there is a single voice stream, this is more likely to be poor quality in the WAN, than something that can be helped by reducing volume. To reduce the RTP volume, choose a codec, like GSM, or, subject to licensing, G.729, that uses a lower bitrate.

Was this problem solved ? If not, what do you mean by bad audio quality, could you please elaborate ?