@zumatech, if you want to port your grandmother's phone number, if her phone number is a cell phone, then porting it over to GV should be doable if you are thinking about using GV. but if her phone number is a landline, then in a roundabout way (that's NOT always guaranteed to work), you can port the number to a cell phone, and then afterwards port that number from the cell phone to GV if you want to go with GV. if you want to go straight to a sip, then you will have to see if the sip can do that for you. if it was porting from cell to GV, the process may take as short as a day or two, assuming no problems. if going from a landline, it can take a week or longer. be prepared if in any part of the process to settle for having to switch to a new "home" phone number if any porting is unsuccessful and if she really wants to dump the phone company. read the Google Voice discussions forum and you'll get a sense for how long it can take and the problems you may encounter.
oh, another thing comes to mind for me, @zumatech, it's not clear to me if your grandmother has DSL or cable, although you do mention "cable tv", but if your grandmother has DSL and tv service and phone service through the phone company, don't let your grandmother make the same mistake that my dad made where my dad simply called AT&T and told them to cut off his phone which resulted in his DSL being turned off. he thought the DSL would still keep working. i told my dad to wait first to set up internet with the cable company and later after we set up a permanent voip solution then later cancel AT&T, but my dad jumped the gun when he got really mad at some AT&T bill and called AT&T to shut off the phone. i wanted my parents to have some overlap of service to make sure that my parents, particularly my mom, still have a regular phone that worked before we had something else working for them. but with my dad cutting off AT&T before we actually had something else set up, that also meant that i wasn't able to help my parents start the process to port over their old AT&T phone number - meaning they ended up with a new home phone number that they had to tell everyone to change on their records.
like @fuzzyeddie, i also have the OBi100 and love using it and discovering how it works. but for peace of mind, i have to agree that @fuzzyeddie's grandmother may be better off keeping a landline. but for someone like @zumatech's grandmother, if she really wants to dump the phone company, then i would add to what @Dav3yDark0 says about 911. i have GV and Sipgate on my OBi100 for the home phone. GV is useful as being my main "home" number that goes into Sipgate where my Sipgate is used for getting incoming calls from both my "home" GV number and my "cell" GV number and also for providing real E911 service on my OBi. you can look at other sip services like Callcentric for getting E911. also take your time and carefully go over the discussion at
http://www.obitalk.com/forum/index.php?topic=339.0 for setting up the OBi to handle 911 calls. you'll have to pay for real E911 service through a sip. if you put in a different rule where you're substituting a phone number for 911 where that call goes out via GV without any sip installed on the OBi, then you're not paying for anything - but no home address information goes out with that. but if you use a real E911 service via a sip, then it's like $1.90 per month with Sipgate and i think $1.50 with Callcentric. they're like prepaid services where you need money on the account for paying the E911 monthly charge and for any outgoing calls made via the sip.
the other option, instead of getting the OBi for one's grandmother (or other elderly parents or grandparents), is to get the Ooma which is more of a complete package that includes the phone voip service - and including E911 service. my parents were fed up with AT&T. my dad was particularly pissed off at AT&T because of some ongoing billing issue. the best option was the Ooma. there was no OBi at the time when i set them up, but if there was, i felt more secure with the Ooma because it meant less techie work for me to set up and almost doing no ongoing "tech" support on my part. and my dad could always call Ooma for any service related problems. prior to the ooma, i had my dad go through using magicjack and skype before moving my parents to ooma and the eventual unceremonious dumping of AT&T which is what my dad wanted to do for over at least a couple years time.
like @fuzzyeddie mentioning about backup battery and power going out, i had the concern about what if there was a power outage, but that's solved by getting a good backup power supply to plug the cable modem, the router, and the ooma (in the case of my parents) into for electricity. (i also have a backup power supply for my own cable modem, router, and OBi.) but if any power outage lasts longer than an hour or two, and if the backup power supply runs out, then my parents have cell phones that should be charged up and ready to use. oh, if your parents or grandparents are "new" to cell phones and if you get them prepaid cell phones, then you may want to have those cell phones registered on the cell provider's website to get the correct home street address registered to the cell phone number for if the 911 call is made through the cell phone. don't just buy the refill cards from the store, but also register any cell phone online if they are to be used as a "backup" phone for 911 calls.
you can set up the OBi for your grandmother if you really want to, and if she really wants to dump the phone company, but seriously consider your options and what level of peace of mind that you want, and what level of support that you are willing to provide because you may have to do at least some initial setup tweaking, and also setting up GV and a sip, as well as making sure they all work together, as well as the times when your grandmother calls you on the cell phone telling you that the obi (or the ooma or whatever) phone that you set up is not working.