Thanks for your replay. I got it to work with Asterisk 13, testing by typing a valid event code into a phone. Because I am running FreePBX in a VM without much memory, I also tried with earlier (lighter-weight) versions of FreePBX. With identical configuration, Asterisk 126.96.36.199 reports:
WARNING: channel.c:5011 ast_write: Codec mismatch on channel SIP/111-00000002 setting write format to slin from ulaw native formats 0x4 (ulaw)
but keeps running and also receives typed events properly.
Asterisk 1.8.17 produces the same warning, but then crashes.
I will report again when I have had a chance to test with my actual alarm, which is a commercial panel.