MiVoice Office 400 and Draytek router issue

[ner0]

Hello!

I'm looking for some help figuring out a problem which, according to the VoIP operator, is being caused by my router, a Draytek Vigor 2960.
In general VoIP calls work just fine, but there is one particular situation where it doesn't - when calls are forward from an outside source.

I'll explain in more detail:
Let's imagine that my VoIP operator sends me SIP traffic from calls received at this number: 1-800-555-0100
Now, let's say that I have 2 cell phones (CPH1 and CPH2), from random operators, independent from my SIP server.
I take CPH1 and forward all calls to my VoIP #: 1-800-555-0100
Then I take CPH2 and call CPH1 directly. In theory it should ring in my SIP server, the same way it does when it's called directly, but the fact is that it doesn't. It just stays silent for a few seconds until the call is dropped by the calling party.

Inside my LAN, before the SIP Server, there is only a network switch and then the internet router (Draytek 2960).
The VoIP operator guarantees that the problem is the router that is blocking or messing with the incoming packets, which I think is plausible but nevertheless I still am unsure how to solve the problem.

So far I've tried disabling SIP ALG and creating a port forwarding rule from 5060/UDP to the SIP Server at that same port (not sure if it's enough, what about RTP?). I've read somewhere that SIP ALG may sometimes mess with the call headers and cause issues. I also disabled every kind of firewall and DoS mechanism on the router, as well as any kind of packet inspection (SPI). But the problem is still present and I'm not sure what to try next. There's just one other tiny detail which bugs me and raises more questions than answers: the call forwarding scheme, as I exemplified above, used to work 4 or 5 months ago. For this very reason I start to doubt if the issue is really on my side or the router's and if the problem isn't actually coming from the VoIP operator or the SIP Server.

I hope I explained this properly, if not I'll happily give more details.
Thank you!

[ner0]

Okay, this is somewhat embarrassing but it works again after a couple of months of not working at all and on top of that the solution involved part of what I had already tried without success.
I hope I won't be coming back later saying that it stopped working for no apparent reason - at least the consistency of how it works/stops working seems to indicate the issue has been solved.

Depending on the level of my ignorance this will either help or amuse whomever reads it:
It started working when I added a port forward rule for 5060/UDP to the same port on the SIP Server (something that I clearly remember doing a couple days ago and not solving a damn thing).
Everything else is as it was, by that I mean I restored the default firewall rules, DoS defense rules, SIP ALG, SPI.

I've now tried this around ten times, whenever I disable the port forward rule, calls will not go through - whenever I enable the port forward rule calls will work.
The thing that I still find weird/unable to explain, is why regular calls will go through without port forwarding of 5060/UDP (using SIP ALG alone), but when calls are being forwarded by external devices (as per example in op) it must have the port forwarding rule enabled for the call to go through.

Weird...