Perhaps you own a Siemens SX66, also known as "Audiovox PPC-6600, PPC-6601, XV6600 / O2-XDA IIs / XDA III / i-Mate PDA2K / QTEK 9090 / Siemens SX66".
Perhaps you've considered purchasing the Jabra C220s or the Jabra C120s to use with your Siemens SX66.
Well, I just tried it, and it didn't work, but it could if you were willing to mess around with customization. The Jabra headsets have posts that are longer than the SX66 headset jack channel by about 3 millimeters. If you plug in these headsets, they essentially go too far into the jack, past the "sweet spot". If you pull the plug out of the jack about 3mm, things work great, but that just sounds like trouble to me.
If you wanted to weld a spacer, sort of like a small stack of washers or something, to the base of the post, the Jabra headsets would work fine.
Go for it McGuyver, you can do it! (I'll pass)