I don't really think Windows Mobile is a very good operating system for mobile phones. It seems needlessly complicated, a bit like Windows for desktops. But just as I use Windows on my desktop computer in the radio shack because I'm not willing to limit myself to the rather poor choice of ham radio software available for Linux, so Windows Mobile offers the best choice of ham radio applications for a portable computer that happens also to be a mobile phone.
A good example of this is PocketDigi by Vojtech Bubnik OK1IAK. I last tried this a few years ago when I had a Dell Axim Pocket PC, though I never used it on the air. Since then, Vojtech has added support for a number of popular digital modes, so the program supports not only PSK31 but PSK63 and 125 as well, plus RTTY, MFSK and several more. It can decode CW as well.
To test it out I just put the phone near one of my speakers connected to the K3 and turned the volume up loud. This is far from ideal, but I did manage to decode a few stations on 20m this morning. One of the accessories available for my HTC Touch Pro is an adapter for a headset, with separate jacks for microphone and headphones. This would be ideal for connecting to a radio. You could make a tiny portable digimode station using this and an FT-817 or even a PSK-20? You probably think the phone is bigger than it is, but the picture above is more or less actual size.
One annoying feature of the FT-817 - and one of the reasons I never actually tried PocketDigi on the air - is that Yaesu stupidly did not allow VOX in DATA mode when the rear ACC socket is used. Someone sent me an audio VOX circuit but it required an audio transformer that I couldn't obtain. But if I can manage to make a suitable interface it would be an interesting thing to try.
If you don't have a Pocket PC or a Windows Mobile phone you can still try PocketDigi. There is an x86 version that will run on a Windows desktop. It's rather basic compared to most digimode applications, but if you are using a very old and underpowered PC with a low resolution screen it could be just what you are looking for.