Problem with 'silent call'

[ai_bp]

Hello,
I’ve got some problems with Asterisk 16.8.

Long story short. I have a simple dialplan for the IVR system. The main part (and probably here is a problem) is below (2*****16 is the call center number).

[inbound-external]
exten => 2*****16,1,MixMonitor(${UNIQUEID}.wav)
 same => n,Answer()
 same => n,NoOp(inbound-phone-call)
 same => n,Gosub(set-globals,s,1)
 same => n,Ringing()
 same => n,Wait(2)
 same => n(greeting),Background(sound/pl/welcome)
 same => n,Background(sound/pl/press-asterisk-for-voice-bot)
 same => n,Background(sound/pl/press-hash-for-ivr)
 same => n,Wait(5)
 same => n,Set(NUMGREET=$[${NUMGREET}+1]})
 same => n,Gotoif($["${NUMGREET}" > "3"]?greeting:)
 same => n,Background(sound/pl/connecting-with-consultant)
 same => n,Goto(queue-call,s,1(main))

[set-globals]
exten => s,1,Set(TIMEOUT(digit)=3) ;  time between digits
 same => n,Set(TIMEOUT(response)=6) ; max time
 same => n,Set(NUMINVALID=1)
 same => n,Set(NUMTIMEOUT=1)
 same => n,Set(NUMMENU=1)
 same => n,Set(NUMGREET=1)
 same => n,Set(NUMREDIALED=1)
 same => n,Set(EXTEN=1)
 same => n,Set(CHANNEL(hangup_handler_push)=on-hangup,s,1)
 same => n,Return()

[queue-call]
exten => s,1,NoOp(caller in queue: ${ARG1})
 same => n,Set(CALLERID(name)=$[${ARG1}-${CALLERID(name)}])
 same => n,Queue(${ARG1})
 same => n,Goto(end-call,h,1)

Everything running on my PC (Ubuntu 18.04) in VPN and usually working fine.
But sometimes (maybe 2-3 times on every 15-20 calls) when I call to number I hear “dead air” (nothing = silence) and on Asterisk terminal, everything looks fine (dialplan is executed and running normally).
When I registered the number in a simple softphone, everytime I heard the signal so probably it’s not a problem with a service where I bought the number.

I don’t know where the problem is. My ideas:

1. problem with VPN (maybe sometimes the VPN generate problem)
2. problem with Asterisk
3. problem with Ubuntu

Did somebody had the same problem? Any ideas?

Regards,
Bart