SIP registration problem via L2VPN QinQ (cisco call manager)?

Question

SIP

Good day, comrades.
First, I will describe the connection diagram, so that it is clear what we are dealing with.
There is a central and remote offices. The central office is connected to the remote one by means of a provider L2VPN, a trunk channel and the admission of only one provider's Wealan. On both sides, the offices are connected using Cisco 3750 switches.
Since the leased L2VPN does not let through other wilans, except for the provider, let's call it Vlan999, and there are switches rather than routers on the boundaries of the offices, I had to pack my own wilans into Vlan999, thereby implementing the QinQ tunnel .
In advance, not all pieces of iron have raised the MTU to 1536 bytes, with reserves. (although everywhere it says that 1504 is enough)
Everything works fine, our weeds go, wireless too, everything is as it should be.
But cisco phones with SIP firmware can't register to Call Manager. They see CUCM, see their dhcp server, take an ip address, send Register packets, but for some reason do not receive a response. Perhaps it's just that SIP requests do not go through the provider's hardware, although the MTU is also increased there.
On the same remote side, devices with SCCP register and work without problems at all.
SIP phones that cannot register are easily and simply registered if they are connected to the local office of the central office.
If who faced, prompt in what there can be a reason, I will be grateful.
If you need some clarification, ask, I will answer.

Answer 1

Vlan is correctly unpacked and given to the port where is the vulture? put a laptop and check if this is the case. Usually, nothing is blocked in transport, except that they set a rate limit. Or a problem with mtu.

Answer 2

voice vlan on the switches is not configured anywhere? And then, maybe, instead of packing in S-VLAN, something else works? Is there a connection via SIP from a soft client (a la Zoiper)?

Answer 3

Askar A It seems to me that the problem is not in transport, but in routing.
branch subnets reachable with CUCM, where are the SIP phones?