Refresh SIP sessions:
Once you go online with your Cloud Link, rebooting your phones may help them reestablish their SIP sessions with any external servers.
WAN IP Whitelisting:
In addition, WAN IP whitelisting may be required by your VOIP provider. When connected to the MDS Cloud Link, your WAN IP would be as shown in your online portal. Use a "What's my IP" online tool to verify you're on MDS' IP and that it corresponds as listed in the portal. Give this IP to the VOIP provider for whitelisting.
Static IP configuration on VOIP phones can sometimes cause issues. Make sure the phones have non-ISP provided DNS servers assigned, as some ISP vendors block DNS requests originating from other networks (in this case, the Cloud Link network). If you are running your own DHCP server, please check what DNS server is being assigned to your phones.
If you set your Cloud Link network to match the existing network, then your gateway should change so you don't have to worry about the network gateway updating on static VOIP phones or your DHCP server (if you're running your own). If you didn't customize your Cloud Link to match the pre-existing network, then please be sure to update static devices and/or any internal DHCP server to use the new settings.
Optionally, the portal has a feature to add trusted destinations by IP or URL so that the security engines leave this outbound traffic alone. Sometimes, this may resolve the issue.
Some partners will choose to route VOIP traffic around the Cloud Link network and send it directly out through the modem. This is typically easier if VLAN capable switching is in place OR if a dedicated network voice switch and physical network drops (not shared with PCs) are run to each phone.