Download Netcat or Use Telnet or Nmap

nc -v -n xx.xx.xx.xx 1723
is it open?

Telnet
open
to xx.xx.xx.xx. 1723

Use pptpping to test and see if the GRE packets are goign through the firewall.
WHat kind of firewall do you use? Does it support pptp passthrough? some dont


Do you want to use L2TP / IPSEC?

did you tes to see if

UDP port 1701, 4500, 500 are open?

Are you going to use RRAS for you VPN server?
DId you enable pptp or l2tp ports?
You using DHCP or going to create a STATIC pool?