Hi Franceso,
like described in the Article you can also use a PBX User Account for SOAP Initialize.
The underlying transport session (HTTP) must authenticate itself either as user (using the users long name and PBX password) or as the admin user (using the gateway administrator account name and password) to perform an Initialize and any session related function. Note that if you use a PBX user account, the user needs to have at least Group/Call Forwards rights.
You have to add the "Group/Call Forwards rights" to the PBX User in the Column "Rights" http://wiki.innovaphone.com/index.php?title=Reference10:Concept_SOAP_API#Initialize.28string_user.2C_string_appl.2C_bool_.24v.2C_bool_.24v501.2C_bool_v700.2C_bool_v800.2C_bool_vx1000.2C_out_int_key.29
Do you have tested this already?
Best regards Basti