Can't access access free pbx system over site to site vpn

Hello to everyone first post here.

I currently look after 2 sites, my main site has a network config of netmask 255.255.254 Gateway

my pbx server resides on this network with an ip of it currently routes out through its own gateway of (third party router)

everything runs very smoothly from this site and no problems and can ping internally from this site

The issue i am having is we have a second site, network config for this site is netmask gateway

we have a site to site vpn set up between the sites using Dell Sonicwall tz300 routers this is working great and servers are pingable at our main site from the second site, everything thats is except the pbx server,

i can’t ping the server or connect to it over the site to site vpn connection.

as a side note if i use open vpn from the second site pbx system works fine. but the idea being i want to avoid using open vpn and just use the site to site.

can anyone help advise where i might be going wrong?

i suspect routing maybe somewhere but can’t understand why i can’t even ping the server from the second site.