You said you would like to use the ipsec and sip features, both you could solve this way:
- Define which ports use IPSec (by a version of IPSec)
- In PortForward function of ZyXEL (should you be available), open ports
- Take / get another WAN router with IPSec feature and set IPSec connection
It should work.
In a similar way should work alternative VoIp with additional VoIP Gateway.
Unfortunately you have / you chose an ISP that pushes a lot of restrictions.
I'm not sure if you can get a Bridge mode for Internet connection,
then things would be a little easier.