The Grandstream GXP-280 looks like a good phone to use with the portable PBX system I am working on now. It is a cost effective, but feature filled phone. The web site states that the phone with most 3rd party SIP devices, gateways and leading soft-switch platforms. This phone should preform very good for the HSMM PBX system.
GXP280 Key Features:
128 x 32 pixel graphical LCD with support for multiple languages
1 line appearance with FLASH to handle up to 2 simultaneous calls
3 XML programmable context sensitive soft keys, 3 way conference
High fidelity wideband audio, full-duplex speakerphone with advanced acoustic echo cancellation
Dual switched auto sensing 10/100 Mbps network ports
Automated provisioning for mass deployment, SRTP