asterisk 18.17.1 unreachable

I have 4 devices that I connect here local and there is no issue.
I have those same 4 devices connecting from another location across the
internet.

They all boot up, connect and register I can send audio to them and they
play.
- then at times they show UNREACHABLE and I can no longer send audio.
then they come back online again and are OK.

I'm using old chan_sip,  I tried changing qualify to no - that did not
help.

What might I adjust to keep these SIP units alway ON ?

Thanks


Jerry

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