Yesterday I was configuring a site to site VPN/pptp connectivity at a remote location, As showed in the above diagram. All connectivity and functions were working fine. Ping from client to client behind each mikrotik was working fine, clients could see each other directly without NATTING, but strangely SIP/VOIP packets were not passing through. upon running TORCH , I could see the SIP traffic on UDP port 5060 was working but in very low volume , in bits. After running some sniffing, I found out that the problem was with the poor implementation at SIP protocol level on most commercial routers and the fact that this technology is just useful for outgoing calls, but not for incoming calls
After turning off SIP ALG (SIP Helper) ,everything start working .
Either by GUI or via CLI
Goto IP > Firewall > Services > SIP [and disable it]
/ip fi service-port disable sip
Now VoIP calls are working perfectly. Writing this, just in case some one else may face this issue with banging his head on the wall😉