How do they know you're using an phone app?

[Rampage80]

There are websites where you can look up the carrier for a phone number. If it's Rogers, Bell, etc., it's a real phone. If it's some other internet/telecom company (e.g. Distributel), it's voip or an app.

Yep this is correct.

Most free text apps have a watermark that most likely can't be removed, or they can't accept calls. I use an app called "Burner" $5 a month for one line that you can make and receive calls and text with no limit. Also text apps usually have some random area code. I have a 647 number with Burner and you wouldn't be able to figure out that it's a phone app.

I have recently discovered line2 which is $15 a month, not sure if its worth it over the free apps. You can still determine if a phone is from an app even if it has a 647 area code.

Can any of these app accept whatsapp as well? On that note, can you have 2 whatsapp accounts on the same phone?

[funlovin]

On that note, can you have 2 whatsapp accounts on the same phone?

As I recall, that depends on the phone. Last I checked you couldn't have two whatsapp accounts on the same whatsapp install - but some phones support (natively or with an app) the ability to install multiple copies of an app (say for example a personal and a work "account") where the settings are independent from each other. So you install Whatsapp under two different "accounts", and each one can have a separate number/user.