I have not specifically used this codec with the NM before, but g723 is definately supported.

There are two different NM-HDV-1E1 modules and depending which one you have determines the number of Channels available for High and Medium codec complexity.

Take a look at the following information I found in the document:-

http://www.cisco.com/en/US/products/hw/routers/ps274/products_data_sheet09186a0080091f6f.html

NM-HDV-1E1-30

Single-port, 30-channel E1 voice/fax Network Module (supports 30 channels of medium complexity VoCoders: G.729a/b, G.726, G.711 and fax or 12 channels of G.726, G.729, G.723.1, G.728, G.729a/b, G.711 and fax)

NM-HDV-1E1-30E

Single-port, enhanced 30-channel E1 voice/fax Network Module (supports 30 channels of high and medium complexity VoCoders: G.729a/b, G.726, G.729, G.728, G.723.1, G.711 and fax)

If you require G723, then you will only be able to have 12 Channels, ensure that you have configured the appropriate codec complexity.

Hope this helps.

Allan.