[cisco-voip] CM 4.1 and Siemens PBX integration

[Constel]

Hi,

I'm dealing with an issue covering CM 4.1(3) and Siemens HiPath PBX with 
HG1500 (with voip modules).
We have to make it talk to each other.
We're able to call from CM to PBX and vice versa just by adding a PBX like a 
H323 gateway at CM and point a route-pattern to its IP.
The problem is I can't use supplementary services (transfer, conference, 
hold, etc.)

We have tried to put an IP-to-IP Gateway "in the middle" to join both CM and 
PBX h323 legs, but it didn't work too.

The router was configured like that:

voice service voip
 allow-connections h323 to h323
 signaling forward unconditional
 h323

MTP was enabled on CM, also "Enable Outbound FastStart" was marked on the 
gateway configuration.

Does anyone have tried that before?
Any help will be appretiated

thank you
Marcelo