@Isaac, we have gone live with the CTI COMET USB device (this was the initial device purchased) in the restaurant for the last four weeks and have been using it without any issues. As we have two phone lines we require another device, I then decided to go with the modem (cheap kit from Amazon) as lot of people have been using the modems as caller id device. We have not gone live with the modem in the restaurant yet, currently waiting for a another laptop. Although I have been carrying out tests with the modem at home I have not seen same issues as you have encountered. I will go live next week with modem and see how it goes in the live environment.
You can purchase the CTI COMET USB device from below link its £85+VAT (£115 inc vat delivery):
http://www.crucible-technologies.co.uk/products/WEB-COMET
Use attached drivers:
comet_drivers_win7_8.zip (767.8 KB)
And important thing to remember is that Baud rate MUST be set to 1200 manually on CTI COMET USB after driver installation instead of default 9600 otherwise device will not communicate with anything.
Note baud rate is 1200 and No flow control.
Additional Information: