Answer to my own question found in mailing list archive, maybe someone finds this helpful:

http://marc.info/?l=freeswitch-users&m=129680941630897

Testet works OK

2016-07-12 13:03 GMT+02:00 Dominique Jeannerod <dominique.jeannerod@interact-iv.com>:
There is an API call : sofia_gateway_data
You can use it inside a dialplan to get the value of a gateway variable.
Should give you the status.

I tried it, but it is not working on my platform, using CentOS6 packages from the okay repo.
Certainly a compilation question (I didn't take time to fix this at the moment).

​

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