Thanks to brontide for calling Dell.
Dell support has determined the cause of the DTMF issue.

Anyway, I finally have a firm understanding, at least, of the reason for the DTMF issue. It goes something like this:

In modern telephone systems there are two types of DTMF signalling:
In-band signaling and Out-of-band signaling

VoIP SIP communication also uses both In-band and Out-of-band.

The "Internet Call" application only supports In-band DTMF signalling, it seems. In addition, the "Internet Call" application does not support the ability to specify the VoIP codec being used. VoIP calls may use one of many codecs for transport of the RTP voice data. The SIP protocol is only used for initiating the call, or tearing down the call afterwards. The SIP protocol may also be used for DTMF, in which case the DTMF would be "Out-of-band." Since Out-of-band signalling is not supported by the "Internet Call" application, a call that needs Out-of-band DTMF signalling will not be able to use it.

Different SIP providers only support certain RTP codecs for certain calls. Often, however, this can be overridden if the SIP client supports the ability to specify the codec. As noted above, however, the "Internet Call" application does not support the ability to specify the VoIP codec being used. Therefore the call will default to the RTP codec specified by the SIP provider or the call endpoint. Fortunately the "Internet Call" application supports most common VoIP RTP protocols, so the voice call data itself usually goes through fine.

Unfortunately, problems with DTMF arise in the "Internet Call" application due to its inablity to specify the RTP codec, along with its inability to support Out-of-band DTMF signalling. Certain common low-bandwidth VoIP codecs, such as g729, simply do not have the bandwidth to support In-band DTMF signalling. In those cases, such as when the call is using the g729 codec, DTMF will fail. It is very likely that DTMF will fail in the "Internet Call" application for all calls using the g729 codec. Unfortunately g729 is one of the most commonly used VoIP codecs.

I don't know what Nokia changed in "Internet Call" from OS2007. Possibly they added g729. Possibly they made g729 a higher priority codec over G.711, which is a much higher bandwidth codec which supports In-band DTMF signalling. Unfortunately, whatever the change was that was made, the "Internet Call" application does not support the ability to specify the VoIP codec being used, nor does it support Out-of-band DTMF signalling. Therefore DTMF in certain calls, ( like my voice mail ) will fail.

So, as is common with many (most, actually) Linux VoIP applications, what we have here with "Internet Call" is not actually a bug, but simply another sad case of problems due to the lack of a full VoIP feature set.

Who would have thought this would occur with one of the worlds largest telephone handset manufacturers?

Unfortunately, since this now seems to be a bug that's not a bug, I wouldn't expect anybody here to hold their breath waiting for it to get fixed. . .

Thanks again to brontide for the Gizmo workaround idea, and the Dell Support call

ps I hope Gizmo5 DTMF works!

Update: Gizmo5 DTMF does not work
Only current VoIP alternative with functional DTMF is SkypeOut.

From the Maemo: Developers Mailing List Archive for the release of the OS2007 Internet Communications SoftwareUpdate for N800

Funny, DTMF actually worked for me back then. . .

Here is another possible workaround for the DTMF problem:

Install Twinkle under Debian, or recompile it for Maemo. I tried DTMF with Twinkle in Debian and it works fine! Unfortunately, the VoIP audio in my Debian version does not work fine (too much latency). That makes Twinkle useless.

In Maemo the audio functions are accelerated by the DSP. I don't think happens in Debian, unfortunately. Twinkle is easy enough to install in Debian with "apt-get install twinkle." It's too bad that the audio doesn't work.

I'm going to try Twinkle under Debian as a chroot, maybe that will work better, as likely the only other alternative is a fresh Twinkle compile for Maemo in Scratchbox. Unfortunately I just don't have the time right now for Scratchbox. Maybe somebody else might be up to the challenge.

Also , a possible fix for me is if there is an easy way to remove the g729 codec from Internet Call, or at least make it a lower priority??? That would fix my situation. If anyone knows a way to do that your help would be greatly appreciated.

Can you post one or two numbers that don't work for you so that others can try workarounds?

Thank all for the great discussion. I tried a few things after reading your posts.. What I found is similar to what you two discussed.

test 1
1. have grandcentral to initiate a call.
2. "internet call" is automatically started with an incoming phone call.
3. DTMF dialing doesn't work.

test 2
1. start "internet call" manually
2. Dial a phone number using DTMF dialing
3. DTMF dialing works

test 3
1. start gizmo
2. have grandcentral to initiate a call
3. Gizmo rings with an incoming call
4. Gizmo doesn't even provide a keypad for people to press a number during phone call.

Only test #2 works. However, this is not a free call..

2 options... 800, 866, 888 numbers are free direct dial through Gizmo. There is also backdoor dialing which is limited, but free.

If you are using Google's GrandCentral Click2Call service, you will be unable to transmit DTMF tones during the call. GrandCentral responds to DTMF tones, but blocks (strips) those tones so they cannot be sent onto to the recipient. What this means is, if you call an automated system that requires that you transmit, for example, "press 1 for English, press 2 for Spanish", you will be unable to comply. Same thing is true for when you use Click2Call to call any number which requires you to enter DTMF tones in a front end menu such as, "enter the extension of the person you wish to speak to now".

Grandcentral does this intentionally at this time. It is some security thing so their network isn't hijacked to route millions of free automated calls, or something like that. It is not a bug, it is not an Nokia N8xx issue. I use GrandCentral and Gizmo with Click2Call everyday with various hardware here. I have a Sunrocket 6011S (actually several) to place free outgoing calls. When I need to call someplace which requires me to input DTMF tones, I do not use Click2Call, I use another (paid) method to make the call.

When GrandCentral goes live as as a "pay" service, I fully expect the DTMF to function, but then the outgoing calls will no longer be free.

If you ask me, I hope GrandCentral/Google leaves things just as they are for as long as possible. I love it just as it is.....free.

On another note:
Kudos to GizmoProject too.... their Gizmo5 "Backdoor calling" is awesome, and Gizmo service has many many other sweet features. Just recently they (Gizmo) expired my calling minutes because I had not used them all, but an email explaining my dissatisfaction resulted in a credit to me. I got all my minutes back and they didn't even tell me they were restoring them, they just did it. Not bad.

~~~~~~~~~~~~~

Nokia N8xx / GrandCentral / GizmoProject = an awesome combination !

~~~~~~~~~~~~~

If you need to place a call then transmit DTMF use another manner, but if you place calls where no DTMF is required, you can't beat GC and Gizmo !

M5

I installed a great application for my N8xx the other day. It is called "DialCentral". Written by Eric Warnke (a.k.a Brontide) and Ed page.

DialCentral is an application which streamlines when I use my Nokia N8xx with the GrandCentral Click2Call function and GizmoProject's SIP service.

1) Have your N8xx connected to the Internet. Have Gizmo and DialCentral installed and running on your N8xx (OS2008 required as of this writing).

2) Enter your GC credentials into DialCentral (only necessary the first time).

3) You key in the phone number of the party you wish to reach on the touchscreen of your Nokia N8xx Internet Tablet, then press "Dial". No stylus is needed because the DialCentral buttons are nice and large.

4)The DialCentral application causes GrandCentral to callback your Gizmo Account (GC Click2Call). You answer the incoming call to your Gizmo on your N8xx and very shortly thereafter you are connected.

Future DialCentral usage does not require reauthentication...just dial on the DialCentral keypad to initiate calls.

It is GREAT ! My Thanks go out to the developers of DialCentral.

M5

(NOTE: Don't use this if you intent to check your voicemail system for messages or call companies/firms with menu prompts. GrandCentrals C2C doesn't allow it. As stated above, this is not a limitation with the Nokia N8xx or DialCentral. GrandCentral blocks the DTMF tones at this time. If you must call a recipient that requires you to press DTMF tones to "get thru" you'll need to use another means. Gizmo DOES work for that, but you'll dial direct with Gizmo at 1.9 cents per minute as of this writing.)

I didn't know about this free 800 calls thing. I also just found out there is actually a way to press a number during a gizmo-initiated call. Since most of my calls to a place that has a phone tree is also a 800 number, this is quite useful.

Don't forget z2n... without him there would be no DialCentral as I probably would have never gotten around to doing a UI!