A special situation arises when you configure 2 or more destinations where your Inbound Calls should forward (Simultaneous Ringing). If you are forwarding to multiple destinations then a number with an extension effectively disables the Simultaneous Ringing feature. This is because the PBX for the number with an extension will immediately answer the inbound call to ask for the extension, and by answering it stops the other numbers from ringing. So the other configured numbers may never be answerable for inbound calls.
When adding phone numbers to the “Forwarding Destinations for Inbound Calls” screen it is strongly recommended that you do not enter numbers that require an extension. A number with an extension is OK to use if you are forwarding to a single destination.
If you need to configure your inbound number to forward to a number with an extension then you should set that number to be the only forwarding destination and not make use of Simultaneous Ringing.