Question: We have an ADSL line which connects to a preconfigured router which is then plugged into the PBX. They are using SIP for incoming and outgoing. So would it be best to have the firewall in place between the router and PBX?

Answer: Yes, in this case it is best to place the VoIP Firewall between the router and the PBX. The main reasons for this general recommendationation are a) to ensure the SIP traffic can be seen (ie. not encrypted and not encapsulated) and b) to optimize the amount of traffic the firewall needs to analyze.