|You have an internal (inside your home and similarly office) ip address; that/those is/are assigned automatically by your router (unless you manually set those addresses).|
Your actual ip address from the www side of things, i.e. your actual ip address onto the internet, is actually quite different; and that is assigned by your isp. As previously stated, unless one pays for a static address you will have a dynamic address - and that can (and will) often change at any time... An "internal ip address" is translated by software within the router so as to appear as that dynamic address (supplied by your isp) to others viewing/accessing your computer(s) from across the internet.
The error messages would seem to imply that either the remote access option is not enabled on computers various (those you wish to use that way); or something is blocking access - possibly the firewall?
I'm not familiar with the ins/outs of VoIP - know what is is of course but have never actually used it or set it up. Possibly there is a setting there that is interfering with your remote access; can you temporarily disable it and see if remote access then works OK? If it is VoIP causing the problem then we have to discover exactly what it is that VoIP is doing in that regard...?
This discussion seems to orbit around a not unsimilar situation to yours...:
Note that although it is correctly pointed about that internal addresses are translated when going out on to the internet, there appears to be some suggestion that if host and client have the same ip address - then problems may arise... And to avoid that possibility one can simply re-assign ip addresses at one end so as not to have the same in use at both ends (host and client). To do this would mean you set say the home system to manual DHCP and use a range of address higher up the range you actually share...
Both home and office use (typically) 192.168.x.y where x is frequently either 0 or 1 - can be higher...; and y is anything from 1-254 (actually anything between 2 - 253 as .x.1 and.x.254 are reserved...).
So if you are happy to... perhaps assign (manually set) the DHCP to give higher value ip addresses for "all" your home kit... and leave office stuff on automatic DHCP assignments. Perhaps all home kit starting at x.50 and on from there? If your office was on say 192.168.0.1 etc... and your home was on 184.108.40.206 etc... then that possible conflict wouldn't arise and thus no need of going manual at one end of the chain?