Loosing my home telephone number would not be my end-of-the-world scenario, but, loosing a business phone number just might be. In the event that my VOIP service provider goes out of business, without sufficient notice, how would I 'rescue' my telephone number and port it to another provider?
a) If your provider provides notice they are going out of business, you should be able to port without issue.
b) If they don't provide notice and just shut down, it depends on whether they are current on paying THEIR vendors. The numbers are held with CLECs (Competitive Local Exchange Carriers), not with the VoIP provider. If the CLEC hasn't been paid, they COULD decide to immediately reassign the number to a paying customer. More likely, they're interested in getting you to stay a paying customer, and they'd be willing to transfer the number to another VoIP provider that they serve.
I don't know how "fancy" your planned usage is, but from my non-expert assessment of the industry there is no reason to pick an "iffy" provider, you're already saving a small fortune vs. your former landline provider, so if it's $.01 or $.015 a minute, that's chump change - and GV is free for US and Canada calls.