Yes, zenfone 2 has serious call drop issue. Call drops in 4-5 minutes. I got replaced my unit after reporting call drop issue, however since the replaced one was also having same issue hence finally requested the refund and got it.
As I observed, call drop happens when network switches between 4g/3g to 2g hence everybody will not experience it, especially those living in strong 3g/4g signal area wont experience it as there is no network switching. However wherever 3g/4g signals are not strong enough the phone try to switch to 2g and call drops. This is my observation and lot many pupil had same experience from India. Just go to Asus forum, there are lot of pupil complaining of call drop/network problem and there is no permanent satisfactory solution available so far. Asus has not admitted it and buyers are suffering. Many have returned there devices and got refund and replaced. I think asus is not bothered about their brand value. I would not recommend to buy the device unless the issue is resolved, which is a basic expectation from a phone.