Regarding cell phone manufacturers: that's not really the same thing.
Phone numbers are universal. It's not like an iPhone user on AT&T can only call other AT&T users[1], or that T-Mobile customers can only call other T-Mobile customers. Skype limits me to reaching out to other Skype users. FaceTime only works with people who have bought into the Apple ecosystem.
[1] Although one might suggest that these iPhone users can, in fact, call no one.
Well, that's due to rigorous standardization. Think about the speech codecs etc. used in mobile telephony. It's not like a phone number gives you anything more than an IP address does.
Phone numbers are universal. It's not like an iPhone user on AT&T can only call other AT&T users[1], or that T-Mobile customers can only call other T-Mobile customers. Skype limits me to reaching out to other Skype users. FaceTime only works with people who have bought into the Apple ecosystem.
[1] Although one might suggest that these iPhone users can, in fact, call no one.