Can't hear incoming calls delivered on SIP trunk

[DonB]

My system has been rock-solid stable for many years. In the last month or two, I've been having problems with not being able to hear external incoming calls (SIP delivered), though the caller can hear me. I'm not aware of any changes, and the firewall test passes fully. I've found that resetting the SIP Server service (including the dependent services) appears to cure the problem. But maybe within a few days it returns. Any ideas as to where I should look to find the cause ?
Thanks,
Don
3CX On-Premise, Debian OS, Standard Annual, Ver 18.0 Update 5, Build 418