Short answer, you can't tell... I'm guessing that most of the residential voip numbers are likely former physical POTS numbers, 

Longer answer,,

Get them to ask a couple of the bigger players for a ball park figure on their customer numbers, - tell them its for a school project, yaddah yaddah,  

🙂

Question is to open to be able to be answered.

Do you want to count managed on a SOHO router customers as voip even tho they probably have no idea what it is or what it means? They just pay one for a phone and it comes from the box. Or it could be cellular on a 4g router. Customer has no idea what it is so does it even matter?

People that use skype to call phones are another one. Its just software to them. Not really voip as most people think of it, but there is voice and IP so perhaps you count it.

Then there is customers on copper thru the cabinet. Not PSTN back to a dinosaur, and its using an IP backhaul but those people have no idea what they are buying.

I think, without getting too esoteric, the question is how many people essentially 'swapped' their copper-based landline for VoIP set up .... so probably excludes 4G etc.

Like wellygary says, maybe best to just call the telco/ISP (Marketing Manager or similar), and see what response is.

I have. So 1 so far.

@huckster .... ha ha ha ... me too, so 2?

Pretty much anyone who has had fibre installed and still has a "landline" phone will have been converted to VOIP.  There are areas where there is enough fibre that the copper is scheduled to be terminated, as everyone can be moved onto fibre if necessary.  It is almost always cheaper to have fibre and VOIP for a landline even if you are not using the Internet, so most people do not complain if they are asked to convert to a fibre/VOIP connection because their monthly bill will typically be at least $20 lower.

Me as well, so at least 3.