Landline on mobile is probably the most robust/bulletproof inbound solution.
If she is willing to push the boat out technically a bit further, Skype is a good option. I had a similar set up when I was working for myself, but ultimately ditched it since no-one ever called the landline number. Benefits include familiarity (for most) and can have mobile and desktop client. Forwarding numbers works well, but you can obviously use the mobile client directly. It's been a couple of years since I did this; then the mobile client was very ocassionally flakey and the odd call went to voicemail. That was absolutely fine in my industry though.
Slightly wider topic, but I remain amazed at the value for small businesses Microsoft offer via O365. That includes a basic CRM with Skype integration (I'm a firm believer that the CRM should be the most important system for every small business.