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Cisco H323 MGW and DTMF relay behaviour


Just want to have a technical position related to MGW H323/DTMF

The Cisco H.323 MGW sends its DTMF capacities in the H.245 Terminal Capability Set (TCS) as described in the general configuration (under voice service voip).

The result of this is that alphanumeric and signal capacities are always advertised.

Instead, we think it should advertise the capacities configured on the dial-peer used for the call.

For example in a call involving the following dial-peer, it should advertise the DTMF out of band alpha capacity, but not the signal one (because h245-signal is not included).

dial-peer voice 221 voip


dtmf-relay h245-alphanumeric

Is it possible to have this behavior ?

Thanks for the help

Community Member

Re: Cisco H323 MGW and DTMF relay behaviour

Hi -

Yes, that it possible. If you do not include h323-sig in the outbound dial peer it should not negotiate that parameter. Remember that a dial-peer specific command will overrule the global setting (in the 'voice service voip, h323' global config section)

you can check with 'show call active voice | inc tmf'



Community Member

Re: Cisco H323 MGW and DTMF relay behaviour

Thanks for your feeback, but actually this is not what happens with our H.323 gateway (IOS 5300 Software (C5300-IK8S-M), Version 12.2(15)T14)

Whether we have h245-alpha or h245-signal on the dial peer configuration or not does not influence the gateway behavior: it always sends both the signal and alpha in the H.245 TCS (terminal capability set).

To my knowledge, there is no global setting to suppress the support of either alphanumeric or signal DTMF exchange mode, like there is for the in-band nte mode (with the h245 caps suppress nte command).

Are you aware of any fix that would have improved this behavior on a more recent release, and if so could you please provide its reference ?

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