Thanks for the advice.
Tried this, but the problem is more complex then I thought. Plainly, sometimes it works, sometimes it doesn't, and I can't understand why.
Can you tell me, what exactly does SIP helper do?

it's working with me without any problems.

What exactly is working? Do you have the same setup?

Hi everyone. This is sad, but the problem still exists in 6.12. My router works as - internet router with NAT - PPTP client with another NAT I have 2 SIP connections - first one to internet service, second to private IP switch on the other side of PPTP. PPTP connection takes some time to establish, ...