How Can I Ensure the CODEC for Calls to my DID?

To ensure the CODEC for calls from DIDWW you need to set at your system the desired CODEC as the only CODEC for this connection.

For example, if you wish to use ULAW CODEC only (on Asterisk) you need to configure a trunk to DIDWW and make sure that the following lines
are added to the trunk:
disallow=all
allow=ulaw