I recently switched my Internet Service Provider (ISP) from AT & T to Wide Open West (WOW!) which switched my modem from 2WIRE to an ARRIS DG860 modem.
Unfortunately, a side effect or consequence of this change was that my Vonage service immediately began experiencing problems namely that I could no longer hear a ring tone on outgoing calls.
After hours of speaking with 2 different Vonage customer support representatives, I did not figure out that the ARRIS modem has a firewall setting which disrupts my Vonage VOIP service. Therefore, I decided to turn off my firewall settings as depicted below on my ARRIS modem to see if I could get it to resume normal Vonage operation and instead rely on my Vonage VPortal firewall settings to protect me from external attackers.
Once I disabled my ARRIS modem firewall settings, of course my Vonage service returned to normal operation!