We are using GXP 2124. The first firmware (1.0.3.xx) that was delivered (and possible still is being delivered) had a problem that made provisioned phones rebooting indefinatly.
Additionally that firmware had a problem that prevented upgrading to the new fw (1.0.4.xx). In order to upgrade these you need to enter the IP adress to the firmware-server rather than the IP name. firmware.grandstream.com will not work on phones with 1.0.3.xx you will need to enter the corresponding IP adress instead. (For security reasons I encourage you to look up the IP yourself rather than to direct you to it.)
After the upgrade they work nicely with 3CX. If you provision them, 3CX will replace the Grandstream logo with their own. That would be OK if the 3CX logo had not been twice as big and owverwriting info in the screen.
/Uffe