This article clarifies whether there is any setting to allow/restrict SMS delivery to VOIP numbers.

• Multi-factor Authentication
• SMS factor

Currently, there is no way to block VOIP protocols for SMS messages. Okta has no way to determine whether the number belongs to a VOIP or not. Okta can only apply restrictions via IP. For more information about how restrictions can be applied based on IP / Network zones, please refer to the Related References section. 

Related References

• IP Zones
• About Network Zones
• Create zones for IP addresses