Why might calls from your SIP device to PSTN devices be blocked?

The reason calls from your SIP device to PSTN devices may be blocked due to non-payment to the Internet Telephony Service Provider (ITSP) is that ITSPs typically require customers to maintain an active payment status to provide services. If payment is not made, the ITSP may restrict or block routing calls from the SIP device to the Public Switched Telephone Network (PSTN). This aligns with standard business practices where service providers enforce payment obligations to avoid service disruptions and ensure continued access to telephony services.

In this scenario, registration of the SIP device, internet connectivity, or maintenance activities of the ITSP could also potentially impact calls. However, these factors are distinct from the direct consequence of failing to pay for the service. A lack of internet connection or device registration would typically prevent calls from being placed at all, rather than blocking them specifically to PSTN devices. Additionally, maintenance may lead to temporary outages, but it does not inherently imply a blockade based on unpaid bills. Therefore, the most relevant reason for calls being blocked in this case is the failure to meet the payment requirements to the ITSP.