OK
Forget routing and forget the VPN for VOIP. If you want it for file transfer that is fine.
I have my family all over the world connecting to my PBX for calls with no vpn or routing.
Connect each device (ATA, Softphone, SIP Phone) directly to the external ip of your office PBX. Use port forwarding on the office router to make sure there are no NAT problems that end and turn off Stun if you can.
Make sure on the gateway firewall in the office that you allow all the necessary ports to be open and forwarded (not translated) to the pbx (use the firewall checker).
The only thing to remember is that if you have a multi extension ATA (such as a grandstream 4 or 8 port) then you need to change the port it registers to 3cx by 1 (e.g 5060, 5061, 5062 etc.). This is because on your (beach house) network the ports all share the same lan ip (e.g. 192.168.1.101) so have to use a different port. This is not the same as having 2 separate phones or ATA (e.g. softphone on the laptop and an ATA) as they are 2 different internal ip addresses and the NAT in your gateway will take care of it.
If you do want more than one analog handset or phone number in the beach house i recommend a multiport (e.g. 4 port) ATA as it is easier to configure.
I have had success with external connections using an PAP2T, SPA2102, SPA3102, and Grandstream GWA4004 behind NAT devices remotely.
Also, what you were missing with routing is that you were routing in the wrong place. You were routing on your client, which doesnt know nor care. It has its route which says send it to the gateway if it is not on the LAN. The router also had 2 routes in it. One for the VPN as you told it the 10.0.0.0 network was at the end of the VPN and one that said send everything else that isnt on this lan or the vpn network out via the internet.
The problem was at the other end. You had no routes in your windows server to tell it to route anything with a 192.168.1.0 network via the vpn connection from your wrt. So it tried to send it out to its gateway which is your internet model (dsl or cable modem probably). This could not send it as it is a private address and so dropped the packets. Hence no audio.
Rather than spend a few hours explaining how to set up routing correctly and how routing works, it is easier for you (as you only want to connect one or 2 handsets to the pbx) to just do it as an external connection as mentioned above.
If you pick an ATA or gateway such as the grandstream or have a phone that is supported, there are a number of guides in 3cx to tell you how to set them up as external connections. Treat your parents to something like the Yealink T20 and it can connect using the instruction on the 3cx site and will work with no vpn needed.
