LTE and VoIP

It's not quite true to say that all 4G calls will be VoIP - true for WiMAX if you count that as 4G, but with LTE (successor to UMTS/GSM), it's still not clear exactly how voice will be handled. Some LTE operators will use "circuit switch fall back" to 3G for voice, some will use VoLGA ( http://findarticles.com/p/articles/mi_m0FGI/is_6_20/ai_n3... ), and others will go IMS (VoLTE), implying VoIP calls - http://www.pcworld.com/businesscenter/article/189339/mobi... and http://www.networkworld.com/news/2010/021510-gsma-one-voi... have some discussion, but IMS is notoriously complex to deploy (it's been the "next thing" for about 10 years now).

Basically, 3G voice fallback has issues with the time to switch networks, while VoLTE is the long term solution (but has had to add SMS support to IMS), while VoLGA is the interim solution using GAN (Generic Access Network, aka UMA) - essentially circuit switch calls over an IP infrastructure, also used today by mobile phones that switch between GSM/3G and WiFi while on the same call (e.g. Blackberry and some Nokias), bypassing the need for femtocells in some cases.

Ultimately it looks like everyone will use VoIP with LTE, but it will take a few years given that some of the VoLTE standards aren't yet finalised.