DIDs or incoming numbers are typically configured with 2 simultaneous incoming channels. In some countries, a third incoming call will be on hold until one of the 2 assigned channels is freed; otherwise, it will sound busy. If you have higher call traffic, you can request additional channels.

At no point do the outgoing lines get occupied, as outgoing calls are handled independently. You can use the number of outgoing channels as extensions that your plan supports; incoming channels or DIDs are solely for incoming calls.

For ITBX, this anomaly is usually caused by limitations in bandwidth availability or the stability of the internet connection used, whether it is WiFi for your smartphone, tablet, PC, or wired network. ITBX communication is fully Voice over IP (VoIP) and requires stable transmission media from server to client to ensure quality.

It depends on the type of configuration. If the call is redirected to your mobile through the SIP client installed on your smartphone, it will use WiFi or data plan connectivity, which is entirely VoIP transmission, so it will not consume your plan. However, if the call is redirected to a mobile number, the route transferring it will use your minute allowance.