Are there any USB devices that can be connected with a Linux Softphone?
These USB phones are enumerating as USB composite device: sound device and some control device, usually custom HID.
Audio part should work with virtually any softphone (e.g. command line baresip or pjsua). One I have (EX-03) has not the best sound quality as it supports only 8kHz sampling, but in general they should be fine for telephone calls.
Control part (receiving events from keybord, using ring speaker or display if present) may be tricky to handle as it may be impossible to find any documentation. If you are lucky you may find other software (e.g. skype control software included on CD) that works with this particular USB phone and then use USB sniffer to analyze packets / protocol.