I've been thinking. (Scarey stuff)
How is your SME server setup? Server Only or Gateway?
If gateway, there must be an option on the extension setup side that allows remote access. I use SARKPBX/SAIL and it has an option for local or remote.
also, may be check that your ADSL modem is forwarding the correct ports 4569 and I think manybe 10000-20000. there would be documentations somewhere. I guess google might be a good option here