I followed some bugs/missing features in the phone-app of the N900 for the last month's and have seen a lot of confusion around what can be expected from the N900 phone part.

A standart feature of GSM is its service codes. This is implemented in every phone I ever used. The *# codes mainly consists of USSD (prepaid, carrier specific services,...) and SS (call diverts, call forward,...).
[NOTE: *# Call diverts, Forwards, etc are NOT USSD, but SS]

So it also would ease the situation of missing flexibility in the standart Maemo. Fortunately this severe issue with the requirement engineering of the phone app was tackled with great .deb's for USSD and diverts from the UI.
BUT in order to consider this 3GPP compliant it needs to be integrated into the phone and even more important into the dial function of the phone -in GSM terms MMI.

Meaning, wherever you put your specified #* code as a number in the phone and press dial the same action is triggert.

Now, not everybody getting involved with the N900 and Maemo is used to read quite scattered 3GPP specs to file a bug !
To MOST users #* are those "USSD things" which is obvious in the threads here and in the bugtracker.

Since USSD is promised to be integrated into the next release the situation with the other *# codes is worse.
Although the bug "Dialer does not accept SS (3GPP TS 22.030) dial strings" was marked as fixed, only ONE *# command was implemented where the spec is about nearly everything the is *# excluding USSD.

To me the USSD and SS services are a related topic, but with existing workaround.
If I see comments that SS are not planned to be supported in Maemo5 and BUG 5357 and 5380 are getting closed, people get banned because they didn't get a good visibilty on what's going on here.
I wonder what the motivation behind semi-open source phones for Nokia is.
What I have seen in the bugtracker so far on this, was more resistance than a community process. At least on things that were "forgotten" by Nokia.

There are a few comments here about the different teams and what the internal fights between them are (actually, how could a phone company "forget" about USSD and SS ??) but paired with the resistance of the team to upgrade the phone which has some type of "coolest toy" status to a real phone (for people who really use it as such) I need to wonder what the motivation behind this is.
Especially in the bug area they expect form the !enduser! knowledge and prescision they even don't have.
(very hard to not get cynical when doing their requirements engineering by investing time and thoughs and get ignored)

So why is this happening?
Nokia's worst issue is software. And this is not specific to Maemo.
Indeed it is needed for Nokia to catch up fast. A working community has a huge potential to ease that situation. To me it seems like the current strategy is trying to get the best of both worlds.
Catching up in software without beeing responsibe (read: COSTS) for it. This can't work if you don't put money in the system. A community needs to be fostered and I really can't spot the mindset yet. BTW: When Nokias plan is to transform into a "services company" I hope there will be a bit more tracktion.

Hope this helps a few when getting into *# troubles and are wondering why the N900 as some rough edges.... and probably I'm not alone wanting a phone that has such a great potential which I can actually USE in business ....

Missing USSD discussion: http://talk.maemo.org/showthread.php?t=32878
USSD widget :https://garage.maemo.org/projects/ussd-widget/
Call Forwarding: Extras-devel repository: Utilits> callforwarding