Yes it's right we can create settings on dialpeers, but I don't have any dsp resources and PSTN provider accept only g711alaw. So I am try to find way how to change default SIP dialer codec from g711ulaw to g711alaw.
As per my Understanding, there is no Default Codec with Sip Dialer.
This configurations are all dependent on Voice Gateway, If you configure Voice gateway to use G711alaw(Based on Dial-peer) the dialer will use that codec.
there is a default codec for Cisco VG, and that is g 729. but you can avoid using that with your setting.
i think you are concerned about codec conversion, so if all will be in single codec you will not need any Transcoding, But definitely you will need DSP for Converting Voip traffic into TDM and Vice Versa.