Configuration Firewall / Voip hardphones and softphones on out network to use with a VOIP Service provider

We have a small network with 5 voip hardphones (SNOM 300/320) and Softphones (X-lite installed on every workstation). We have a firewall/vpn router D-LINK DFL-200. We subscribed to a VOIP Server provider (see [login to view URL] for more information about this service) and we need to configure and use all the hard and softphone with this provider, but we are having problems in configuring the firewall correctly to work with more phones. We are looking for a very competent person in VOIP who can set up all the needed configurations through VPN in our network.