You should be more specific about your requirements. There are dozens of billing solutions for AS5300/5350/5400, starting from free (Trabas http://www.trabas.com/opensource/products.html), hosted (no ownership costs, but you pay all the time for usage – e.g. http://www.ipcb.net), up to middle-range (e.g. http://www.portaone.com or http://www.sysmaster.com/p_vp_billing.htm) and to the deluxe ones (e.g. http://www.portal.com/)