GV is great but being a free service, when things go wrong, support can be sketchy to non-existent.
You might consider using both GV and
voip.ms. Port grandma's number to
voip.ms for free during their current special. The DID (phone number) will cost you $1.49 a month. The $20 you save by not paying GV to port will cover 13 months of DID service. And you will save additional cost and grief by not having to port the landline through a cellphone in order to get it on GV. You can then either pay by the minute .0149 cents for incoming calls to her number or get a monthly flat rate incoming plan at $6.95 (which includes the $1.49 for the DID) for up to 3500 minutes. So the break-even between the per minute and flat rate is about 366 minutes ((6.95 - 1.49) / .0149). I'm on the per minute and have never exceeded the 366 minutes. Outbound U.S. calls will cost $0.0105 per minute value route or $0.0125 per minute premium route or use google voice for outbound for free. You can fund
voip.ms with a mininum payment of $25. I suggest paypal to avoid annoying international transaction fee that may apply on a credit or debit card since
voip.ms is based in Canada. Callerid name lookup at
voip.ms will be .008 (8 tenths of a cent) per call but add her frequent callers to her
voip.ms address book and they will be free. You won't get incoming callerid name lookup on GV unless you jump through more work-around hoops that are covered elsewhere in this forum. Less an issue if you have everyone in the phone's address book.
Use google voice number for her outbound calls for free. Except 911 will be be configured to route through
voip.ms. 911 service at
voip.ms will cost another $1.50 setup and $1.50 per month.
The costs above are for my ratecenter. Your numbers may be higher or lower, but probably not significantly.
This setup works well for us. I don't have 911 activated on
voip.ms so cannot comment on it's reliability. Our frequent callers call our google voice number for free. The myriad of other callers, doctors, dentists, etc. that have the former long-time AT&T number continue to call that number and they now come in through
voip.ms. We use google voice for outbound.
It took about 20 days to port the AT&T landline to
voip.ms. AT&T held the number until the current billing cycle ended. We had already paid through this date (pay ahead invoicing), so it wasn't a big deal.
All said and done, you will have a real voip provider (voip.ms) that you can configure and GV for what I will guess is significantly less per month than Verizon. If it doesn't work out you haven't lost much. The port is free (for now) and the
voip.ms balance is refundable.