Om Malik notes another VoIP choice for Windows Mobile devices. X-PDA looks to be specifically tested with the HP rx1950 and hx2004, but I can’t see why it wouldn’t work for other Windows Mobile devices. There are two versions available: one for QVGA screens at $32.68 and one for 240 x 240 square screens at $39.23. I normally stick with Skype on my mobile device, but the one advantage that X-PDA appears to have is that it’s SIP-based, unlike Skype.

X-PDA requires Windows Mobile 2003 or higher and there’s a full list of features on the product page.