The CallCentric VoIP service, which I myself use, provides iNum numbers to their customers for free. Once I set-up my iNum number with CallCentric, I was able to receive my incoming iNum calls via my VoIP ATA or IP-Phone connected to CallCentric.
The point I'm working my way up to here is that CallCentric also provides PSTN gateways to access their VoIP network. And, because CallCentric supports free calling to iNums, I realized that I could also use the CallCentric PSTN gateways as an alternate means of free landline calling to world-wide iNums - just like Voxbone provides PSTN access numbers for free iNum calling.
CallCentric refers to their PSTN Gateway numbers as Calling Card access numbers. This is because once you access the CallCentric network via a PSTN gateway number, you can then make long distance calls to any PSTN landline or mobile for a small fee (1.98¢/minute on average). But, keep in mind that once you're on the CC network, you can then call any other CC user for free, access the SIP Broker Network for free, call any SIP URI for free (using the CC online Phone Book speed dial feature), and......... call worldwide iNums... for free.
So, you may ask, why do you care about CallCentric PSTN gateways, if you can already call iNums via iNum PSTN gateways?
The answer is that CallCentric has PSTN gateway numbers in cities and area codes that iNum (Voxbone) does not have a presence in. So, the idea here is to "expand your options" by which you can make free iNum calls from PSTN landlines or mobile phones.
For example, iNum provides 71 access numbers in 47 countries (at time of writing).
And, CallCentric provides a total of 74 access numbers in 16 countries (at time of writing).
While iNum has access numbers in more countries, CallCentric has many more area codes covered in the USA and Canada. So, if you reside in North America, CallCentric offers more area code numbers for PSTN access.
What this all means is that you, your family, friends, and colleagues, will have many more ways to make free calls to iNums from PSTN landlines and mobiles.