Please make sure that *all* of the following subnets are whitelisted on your firewall and incoming traffic coming from those is accepted:
174.37.245.32/29 (6 IP addresses from 174.37.245.33 to 174.37.245.38)
174.36.197.192/28 (14 IP addresses from 174.36.197.193 to 174.36.197.206)
173.193.199.16/28 (14 IP addresses from 173.193.199.17 to 173.193.199.31)
119.81.44.0/28 (14 IP addresses from 119.81.44.1 to 119.81.44.14)
in your Firewall.
Inbound traffic from Nexmo might come from any of the IP addresses included in the mentioned subnets.