Quoting kai centos@sandsengen.com:
I'm having problem connecting to my isp with my sangoma s518. Everything seems to run smooth and wanrouter are training to connect....but forever..
Check that username and password in /etc/ppp/pap-secrets and chap-secrets are correct. It might seem trivial, but I got burned with it once. Also check that you pointed it to correct Ethernet interface. If the modem is directly connected to the Ethernet interface (in other words, it is used only as connection to the modem) you want to have that Ethernet interface in UP state, but with no IP address on it (well, at least in default config). You could do something like "ifconfig eth0" or "ip addr show dev eth0" to check if the interfaces is up or down (you should see word UP in the flags).
For example, your /etc/sysconfig/network-scripts/ifcfg-eth0 (assuming we are talking about eth0 interface here) should look something like:
DEVICE=eth0 ONBOOT=yes BOOTPROTO=none HWADDR=_mac_address_of_interface_here_ TYPE=Ethernet
Than you'd simply bring it up with "ifup eth0" command (if it was down).