Troubleshooting phones connecting to hosted VOIP on WAN

  • We've just moved offices & switched to VOIP phones, using a hosted provider. i.e. here, we just have the phones & they each connect through directly to the provider.

    So far though, we have had no luck in getting the phones up & running. Our VOIP provider says that the phones are not registering properly on their network.

    In terms of the setup here, the landlord has a leased line & then leases some of their bandwidth to us which we take from one port of their firewall router. There is also some sort of firewall at their ISP's end. We have a static IP address & any ports we request are forwarded through to this from the Landlord's firewall (its a Fortigate device).

    The VOIP provider has given us a list of requirements, which we are all satisfying at our end. They are mainly to do with outgoing TCP & UDP ports, which they need to be open.
    In additional requirements, they also list UDP fragmentation (I've tried changing the settings for this with no effect) & SIP ALG. My understanding is that the default install of pfSense doesn't include SIP ALG, unless the siproxd package is intalled, so I can't see this as being a problem.

    Our landlord disabled SIP ALG on their firewall, so the only place it might still be active is on the ISP's firewall / router (we're still awaiting a response from them).

    I'm not at all sure where to go from here - the VOIP provider say that they have run a trace & the problem is somewhere at our end.

    I've tried the various other suggestions I've found here - of changing the outbound NAT to manual & setting conservative state table optimization, but this makes no difference.

    I'm wondering whether there is any similar way to trace things from our end to try & pinpoint the problem. At present, the phones are just displaying names / numbers, but not giving any dial tone or connecting through to anything.

    Our VOIP provider is Gamma, & the phones are CISCO SPA504G that came pre-configured by our phone company.

    Any thoughts or suggestions would be welcomed.