What Non-VoIP Means Here
A VoIP number is an internet-based number. A Non-VoIP number is connected to a physical SIM hosted by a mobile network. Some services treat those number types differently in their own rules, and a service can reject either type. Acceptance is decided by the service you are signing up to, and that service's terms should be checked first.
If a number you received is rejected, cancel the order before a code arrives and try a different country or service, or contact support at support@smsotps.com with your order details.