Выбирайте сами, что для вас важнее, передача факсов или возможность выбора одного из нескольких голосовых кодеков.

Cross-Platform Release Notes for Cisco IOS Release 12.2 T, Part 7:
Caveats for 12.2(2)T through 12.2(11)T10
http://www.cisco.com/univercd/cc/td/doc/product/software/ios...

* CSCeb01641

Symptoms: T.38 fax calls may fail if the terminating gateway sends fast start
elements in multiple H.225 messages (for example, call_proceeding, alert, connect).

Conditions: This symptom is observed only with T.38 fax calls that go through an
IP in IP (IPIP) gateway and is specific to fast start calls when non-Cisco
gateways are used for the terminating gateway.
The symptom is not observed on a Cisco gateway.

Workaround: Initiate the T.38 fax calls using H.323 slow start.

* CSCea20911

Symptoms: T.38 fax calls fail when FastStart is used with more than one codec proposed.

Conditions: This symptom is observed with IPIP gateways.

Workaround: Use SlowStart for any calls that require fax.

* CSCdz74292

Symptoms: T.38 fax calls from one gateway to another gateway may fail.

Conditions: This symptom is observed when you make a T.38 fax call from
an originating gateway (OGW) that is running Cisco IOS Release 12.2(13)T
and that is configured for H.323 fast start mode through a gatekeeper to
a gateway that is running Release 12.2(11)T2.

Workaround: Configure the voice service voip global configuration command
followed by the h323 call start slow voice-service configuration command
on the OGW.

* CSCin32436

Symptoms: T.38 fax relay calls cannot be made.

Conditions: This symptom is observed when a T.38 fax relay call is placed
between a Cisco 3640 and a Cisco Catalyst 4000.

Workaround: Enable fallback to Cisco fax relay.

* CSCdu79392

Symptoms   An H.245 connection may enter an indefinite loop because of
asymmetric codec negotiation failure with slow-start calls.

Conditions   This symptom is observed when slow-start calls are made
using a voice class codec configuration in a Voice over IP (VoIP) dial peer.

Workaround   Configure the codec directly on the VoIP dial peer with slow-start calls.