Grandstream frequently updates its firmware and may be too fussy for EPM to keep up with their changes. For Grandstream devices, I use their free provisioning service GDMS (GDMS) which gives me tremendous flexibility to provision the Grandstream phones and ATA’s individually or in bulk across systems. It also lets me know if an endpoint is down or rebooted. The GDMS can also update firmware across all Grandstream devices you register with the software.
One more note - I went the GDMS route (Thanks kenn10!) with a new HT801, and while it is pretty self-explanatory, I needed to remember to specify the port number when defining the VoIP server (i.e. host:5160).