I used these with an Asterisk system. I liked the physical design, the feel, the sound quality, and the price. But we soon started experiencing phone lock-ups, intermittent one-way speech (and not just due to problems with the SIP provider at the beginning of a call), missed call terminations, etc. Almost immediately I got to spend time troubleshooting, getting out WireShark and doing call captures to find out why the PBX was not working right, and it all pointed at the Grandstream phones. After several months of fighting with the company that sold us the phones, they agreed to replace the Grandstream phones with Polycom phones, and the problems were resolved.