Where do you change the FTP url? Is it on the VM running debian or on the FTP server? Do you allow IPv6 incoming/outgoing traffic on the FTP server? In general, FQDN resolves to both IPv4 and IPv6.

HI,
I change URL in my 3CX phone system, only software running in debian 9.5 VM.
FTP server is on premise where lots of others phone system do the same FTP backups (fine, with FQDN URL) as I try with this new VM and not working.

On premise I have a router with IPV4 and a port is forwarded to FTP server.

I already got some problems with windows machine having DNS request answered first with IPv6 instead of V4 and got some problems like today , but it was with local windows computer so I knew what to do.
For a cloud machine I'm not able to find what to check and where.

I answer myself if it may help someone else with same problem

From a SSH session do following commands to disable IPV6 on all interfaces with SYSCTL

Do a ifconfig to see if IPV6 is enabled on network settings, if it is then
sysctl -w net.ipv6.conf.all.disable_ipv6=1 (disable ipv6 all interfaces)
sysctl -w net.ipv6.conf.all.autoconf=0 (disable auto config all interfaces)
sysctl -w net.ipv6.conf.default.disable_ipv6=1 (disable ipv6 from default settings)
sysctl -w net.ipv6.conf.default.autoconf=0(disable autoconfig by default)
do once more a ifconfig to check there's no more IPV6 inet6 lines

Last step to write these changes at EOF Sysctl.conf
Edit with nano /etc/sysctl.conf
add these lines:

Disabling ipv6 all interfaces

net.ipv6.conf.all.disable_ipv6 = 1

disabling autoconfig all interfaces

net.ipv6.conf.all.autoconf = 0

disabling ipv6 for all new added interfaces

net.ipv6.conf.default.disable_ipv6 = 1

disabling autoconfig for new interfaces

net.ipv6.conf.default.autoconf = 0

After this is saved in file, reboot your instance , re open SSH, do a ifconfig to check there's no more ipv6 here, done ;)