Me too. I realise if making calls, the phone keep swapping betwwen 3g & 4g networks, hence unable to call out. This problem only occurs when you set network to 2g/3g/4g. Works from on 2g/3g, 2g or 3g network mode.
I'm quite certain that this is a software problem. In 2g/3g/4g mode, the phone works fine in my camp where 4g coverage is unavailable most of the time.
The network needs to be fixed when making calls so as not to experience delay due to continuous handover/swapping of network.
Problem occurs on ze550ml and ze551ml 4gb/32gb version. Eliminated the cut microsim and faulty sim slot theory.
What I have tried:
1) Rebooting device
2) Changing sim card
3) Tried another same model device
4) Cleared device cache partition (See:
http://www.asus.com/zentalk/forum.php?mod=viewthread&tid=9229)
Temporary solution:
Use the other cellular modes except 2g/3g/4g if you need to make calls.
Update:
The phone is unable to call because it does not support volte (Voice Over LTE), hence it needs to switch to 3G to make calls but it goes into a network loop between 3g & 4g.