In general, the ISP can block the TCP 1720 port only, try to :
1. If both VOIP Gateway in two endpoint support modifying the H.225 Q.931 TCP port ( like change to 1721 ), change them;
2. Most VOIP Gateway can’t modify the TCP port of Q.931, try to use some Port Mapping software like :
PortTunnel http://www.steelbytes.com
GDataPipe http://www.gwacc.co.za/software.htm
PortMapper http://www.analogx.com
So it should works like :
VOIP—1720—PortMappingComputer—1721—–Internet—–1721—PortMappingComputer—1720—VOIP
Good luck.