[Serusers] Host three way conference

Hi all, I've some questions about it. First of all, when I use a SPA-941 with SER, I can do a three way conference with two incoming calls. Why I can't if I use an other product like SPA2100 ? I can deduce that this service depends on the functionalities of the product, but I'm not sure. But if I want nevertheless to use this service independently of the product used, how can I do ? Using SEMS ? In this case, can we install it without reinstallating SER ? Moreover, with SEMS I think I will have to determine a trigger ? (ie. something like a prefix in the URI ?) Thank for your answers and your support. Regards, Adrien .L