second line app recommendation

I have been using Dingtone for years now.

First, it is free to get started.
Because you are looking to do business, your useage is different than mine, so a business number and more minutes (credits) are sure to cost more than free.

It can use Wi-Fi as a connection, so your call quality is dependent on the quality of your Wi-Fi.
(It can also use your network connection, but this will use data.)

You can have five devices all using the same number.
A tablet or a PC can be used as a phone.

This is a VoIP service, so it is independent of your carrier- you can have a Dingtone number and your cell number active on the same device at the same time.

This number works just like a normal mobile number- it can make and receive calls and texts to/from any other number.

Your calls can be forwarded to another number.

Credits are pretty cheap to purchase.
The more 'points' you earn over time, the better the deals get.
Right now, if I were to purchase credits, I get 10% more than what I paid for.

Current cost for me, as I check now, are...
200 credits for $4.99
500 credits for $10.99
1000 credits for $19.99

Keep in mind that I did not add in my 10% bonus, and I do not have a 'business account'.
As I do use it for 'business' (I was self employeed for a long time) I kept my talk time low and I have never needed to purchase anything.
I currently have over 14000 credits just from 'checking in' and watching ads, and various other ways to earn free credits.

There are 1-800 numbers available, along with other toll free options for your customers.

Count on calls costing you approximately 1 credit per minute.
(It is actually less than that, at least for me.)
Texts are 1/2 credit per text.