If a trusted caller is coming in via OBiTalk or SIP and calling out on GV, that will work properly, even when you are on an unrelated GV call from the Phone port. The OBi can handle up to four concurrent calls. You can obviously have only one on the Phone port (except for 3-way calling and call waiting). Each GV account can have two; limits on SIP accounts are determined by the provider.
If the caller is coming in on GV and out on GV, in most cases I would recommend not using the OBi at all. He would call in to his own GV account and press 2 at the main menu to make an outbound call. This results in better quality, is more reliable, and doesn't tie up any of your resources.
If you must use your fax machine to receive while on a call, you will need to buy another OBi (or perhaps an IP phone or an ATA with multiple ports). However, you may prefer to get faxes via a service that delivers them by email or web. For me, the big advantage is being able to view faxes from anywhere, on a laptop, tablet, or smartphone. You can save them on your computer forever, rather than accumulating paper files, don't kill as many trees, and can forward the fax (possibly edited) to others, without a generation loss. You can get an inexpensive local or toll-free fax number from Anveo, Callcentric, Vitelity and many others. If the number is unimportant, VoxOx is a decent free fax-enabled service.