Tel HTML through Gmail on iOS

Hello,

i am wondering why my HTML code is not working on iPhone, even though it is working on Windows PCs and Android phones.

I listed a phone number in an email and made it clickable by adding this HTML code: <a href="tel:978-921-4334">978-921-4334</a>

Having sent the email, I accessed it on my phone through Gmail. The phone number is blue and looks like a button, can be clicked, but when clicked, it does nothing. I am not asked if I wish to call the number. This is not the case on my Windows PC and Android phone. The button responds as it should.

Any idea on why this might be the case?

Thanks in advance.