No audio on Cisco phone via VPN

Hi!

My problem is that others I call from a specific Cisco phone can't hear me. If they call me, after a few seconds I can hear them speaking.

Infrastructure:

Main floor - Mikrotik router (acting as a VPN server), 192.168.88.1 - many devices attached to this one, servers, phones, pcs, and machines via VPN from distant places. Reception - TP-Link router with DDWRT connecting to a provider via PPOPE for security camera feeds, and internet access - 192.168.15.1 - VPN passthrough is enabled. Reception - Netgear router attached to the TP-Link router, connecting to the Miktorik router via a PPTP VPN connection, using the Mikrotik router's public IP address. The Cisco phone in question is attached to this router. 

We need the TP-Link router for internet access and camera feed, and we tried connecting through this PPOPE connection with the built in DDWRT VPN client, but it won't work, on the forums people say it never worked, that's why we included the Netgear router.

The connection between the phone and the FreePBX server is fine, all lights are green, phone gets an IP address via DHCP, and can start and receive calls, but if I call someone, they can't hear me.

We have another Cisco phone connecting from another distant place via VPN with a Linksys router with DDWRT (via direct PPTP connection), and it's working fine. Both phones have the same settings except specific settings.

We had the same problem before the camera feeds, when the TP-Link router was connected to the Mikrotik router directly with a static PPTP connection, but before using the FreePBX server, we had an Asterisk server, and the phone was working.

What could be the problem? Because the Netgear router doesen't make any difference I guess it must be the TP-Link one with the ddwrt, or the server itself.

Any help would be appreciated, thanks in advance!