Hi:
Regardless of how you set up the OBi and the phones, you'd need to port both of their telephone numbers to one or more service providers. Note that Google Voice doesn't accept ports-in from land line numbers, and you'd need to go through the exercise of first porting their numbers from their land line provider to a prepaid mobile phone provider, such as AT&T GoPhone or T-Mobile prepaid, wait at least a few days and then port the numbers into GV. If you want to instead use a SIP VoIP provider, most can port in land line numbers directly.
See the tutorial on Obihai's FAQ section for instructions on the land-line-->mobile-->GV porting shuffle.
Regardless of which company hosts the numbers, you will need to have two separate registrations, one for each phone number. The Internet Telephone Service Provider world calls inbound numbers "DIDs". OBi 2xx devices can be configured to host up to four separate incoming DIDs, and you can customize how outbound calls are made (which number is used by default, vs. selecting the other number manually).
You can duplicate the same user experience they have now, with an OBi 202. I assume that your parents do this because they need to have two different telephone numbers, and the cordless phone systems only support one phone line.
If they must keep the current
mess setup of two separate sets of cordless handsets, then you'd simply plug one base station's RJ-11 phone cord into the PHONE1 port on the OBi202, and plug the other base station's phone cord into the other PHONE port. You would then configure the 202 with DID #1 on say, SP1, and DID #2 on SP2, and tell it to ring PHONE port 1 for SP1, and PHONE port 2 for SP2.
On the other hand, this could be an opportunity to make a "paradigm shift"
Using a less-expensive OBi 200, and just one of the two cordless phone systems, you could have the OBi ring one set of cordless phones, regardless of which of the two DID numbers is being called. For outbound calls, you could decide which number to use by default, and then prefix the calls with a star command to use the other number.
For example: if DID #1 is on SP1, but you want to place a call using DID #2's service and caller ID, then you'd prefix the dialing with **2.
Advantage: half the cordless phone handsets to keep charged and waste electricity and replace batteries and find when they get lost.
