Quick overview from the supplier:
Grandstream GBX20 Extension Module, 20 Buttons, 40 Contacts, Black
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supported by the GRP2615, GRP2624, GRP2650, GRP2670
4.3 inch (272x480) TFT color LCD
20 per page (each module contains 2 pages, for up to 40 lines per module)
Combine up 4 daisy-chained modules for 160 contacts/extensions
BLF, call park /pick-up, speed-dial, presence, intercom, voice conferencing transfer/forward and more
https://www.grandstream.com/products/ip-voice-telephony/extension-modules/product/gbx20-extension-module
