[Serusers] Problems using REFER method using SER and Cisco gateway

[Alcor]

Hello All,

I recenlty installed ser stable version (0.8.14) and I use it with a 
couple of Cisco IP Phones and a Cisco 36xx voice Gateway. I installed it 
using mediaproxy because some phones are behind a nat and it works fine 
for "basic calls" (IP-IP, IP-PSTN and PSTN-IP).

However I have a problem when I try to transfer (using attended 
transfer) a call coming from PSTN to a phone that is behind nat. The 
problem is the following:

The phone originating the transfer sends correctly the REFER method to 
the Voice gateway through SER server containing the refer-to: 
number at publicIPaddress but the Gateway sends the INVITE method "directly 
to the publicIPaddress of the NAT", and doesn't pass through the SIP 
server (that "manages" the messages for the NATed phones) even if I 
configured a dial peer voip matching the voip strings and specifying the 
server where I have to send the INVITE requests.

My question is: is there any way to configure SER changing the REFER 
message in order to "force" the gateway to send the INVITEs through the 
server?

Or, anyone knows if something more than dial-peers has to be configured 
on Cisco gateways?

Thanks a lot in advance,
Raffaele.