try as many different modem scripts as you can, search the fora for keywords 'modem script'
btw, i have found that GV external modems are nothing but a pain, and i have way too much experience with them on many different Macs (owned by many different people) in different locations here in the USA... i bought a GV 56K modem some years ago and i got rid of it shortly afterwards, undoubtedly the crappiest modem and modem software i have ever used