Depending on the router, IOS version, and gateway type it may be possible. A good reference for this starts on page VR-188 of the following command reference doc:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fvvfax_r/vrf_a.pdf
In the doc it states the following.
codec-Sets the codec options that you can use when you execute this command.
Codec options are as follows:
?clear-channel?Clear channel at 64,000 bits per second (bps).
?g711alaw?G.711 a-Law at 64,000 bits per second.
?g711ulaw?G.711 u-Law at 64,000 bps.
?g723ar53?G.723.1 Annex A at 5300 bps.
?g723ar63?G.723.1 Annex A at 6300 bps.
?g723r53?G.723.1 at 5300 bps.
?g723r63?G.723.1 at 6300 bps.
?g726r16?G.726 at 16,000 bps.
?g726r24?G.726 at 24,000 bps.
?g726r32?G.726 at 32,000 bps.
?g728?G.728 at 16,000 bps.
?g729abr8?G.729 Annex A and B at 8000 bps.
?g729ar8?G729 Annex A at 8000 bps.
?g729br8?G.729 Annex B at 8000 bps.
?g729r8?G.729 at 8000 bps. This is the default codec.
?gsmefr?Global System for Mobile Communications Enhanced Rate Codecs (GSMEFR) at 12,200 bps.
?gsmfr?Global System for Mobile Communications Full Rate (GSMFR) at 13200 bps.
bytes-(Optional) Specifies the number of bytes in the voice payload of each frame.
payload_size-(Optional) The number of bytes in the voice payload of each frame. See Table 12 for valid entries and default values.
