743 - Entity's credential/enrollment information. Usage: This code requires use of an Entity Code.

This payer requires that you enroll with them before you can submit claims to them.  This rejection can possibly mean one of the following (that we can think of).

• You are not an enrolled provider with this payer.   In this case, contact the payer to find out how to enroll as a provider with them.
• The rendering provider is not an enrolled provider.  In this case, contact the payer to find out how to enroll the rendering provider.

If you are enrolled with the payer, then it could mean that you were not enrolled at the time of service.  The message received from this does state "on DOS".

• You are not submitting your claims using the same NPI that you used when you enrolled with this payer.
• You are not submitting your claims using the same Tax ID (TIN) that you used when you enrolled with this payer.

You may want to contact the payer to find out what NPI and Tax ID they have on file for you and make sure  you use that NPI and Tax ID when submitting claims to them.