Issabel forked from Elastix in 2016 when 3CX acquired Elastix's commercial brand. The fork preserved the GPL-licensed core: Asterisk, FreePBX modules (older fork), call centre suite, email, fax, IM. For operators who used Elastix in production and never want to deal with proprietary licensing, Issabel is the continuity option.
The trade-off is a smaller community than FreePBX, slower module updates, and Asterisk version lag. Issabel 4.x ships Asterisk 16. Production users plan for end-of-life of Asterisk 16 and migration to FreePBX 17 or vanilla Asterisk 20.