Is it possible to dial sub(extension) number after answering PBX.

For example, If PBX is connected the remote voip, can we dial a sub number (extension number) through the PBX.
Yes, if you dial into a remote voip channel that is connected to a PBX, what you will receive is dial tone from the remote PBX. From this dial tone, you can dial any extension/number the users who are local to this PBX are able to dial.