Stepping back to try to address the original post a bit.
To start off, to be able to integrate VoIP and POTS ("plain old telephone system"), you need at least a device that implements an FXO port -- this is the connect to the POTS. To use a "regular" telephone, or fax machine, you need an FXS port. If only using softphones, no FXS device is needed.
If it is all lashed together, incoming calls can be received through POTS or Internet; softphones can dial out across the POTS or VoIP.
As I understand it, MagicJack is primarily a simplified connection to a SIP network, and is not free for out-of-network calls. MagicJack has an RJ-11 outlet, which is operating as an FXS port. It is unclear how much work would be needed to make it readily available for other applications. But, it would probably be non-trivial (hardware drivers). It would be quite a bit of work to make it operate as an FXO port. Trying to make it work as a "switchable" FXS/FXO would be, um, not worth the effort.
From the MagicJack FAQs:
- "Will magicJack work with Linux? Not yet."
- "Does magicJack work with PBXs such as Asterisk? NO, magicJack does not work with PBX systems."
Of course, that all makes sense. The reason why they can keep their prices so low is because of the interconnection fees they charge to other telecommunications operators.
The suggestion to use some FXO/FXS device, such as a Sipura3102 would likely be a better solution. That way you could use a regular handset, connect to a POTS line, choose the SIP or IAX provider of your choice, and your USB port remains clear for other uses.
Of course, if one is willing to give up the USB port, an alternative is something like the Samgoma USBfxo. (I've worked with the Sipura, not the Samgoma.)
(These are not the only two such devices, Google for "FXS FXO".)
This is another case where the solution needs to be dictated somewhat by the requirements.
In the U.S., some of the better prices for VoIP providers runs around $2/month for a DID ("telephone number") and $0.029/minute per call. If you receive an average number of incoming household calls each month it is even possible to get a DID to receive the calls with no charges (monthly or per-minute) whatsoever.
A Sipura is currently around $75, a Samgoma $131. Just need to estimate your monthly charges using VoIP, then figure out how long it would take to recoup the investment, if you reduce your telephone bill from the current level to the estimate.
Just to throw in another wrinkle, Asterisk can also be connected to Skype and Googletalk, adding other free communications channels; MJ won't do that.