Well, if it's a BTS, by definition it speaks Abis. However, Abis is not fully standardized, and every vendor has their own dialect. We reverse-engineered the dialects of Siemens, Nokia, Ericsson and ip.access to support them in OpenBSC, OsmoNITB and today OsmoBSC.
Whoops, Brain blip on my part... It speaks the Huawei flavour of Abis over IP, and not over E1.
There seems to be quite a few bits of documentation on scribd https://www.scribd.com/search?content_type=documents&page=1&query=bt...
Came across this blog too: https://nickvsnetworking.com/tag/bts3900/
Seems they've been experimenting with an larger sibling of this Picocell.
Best,
-Saturn