Just to add my experience, I had the xz premium when it first came out in June last year, pre ordered it as long time Sony user, previous phone was z2 with no call issues.
Had same issue with person I'm taking to suddenly hears clicks then can't hear me for random amount of time and I can hear them an shows I have signal on phone.
Ended up returning phone for 3-4 replacements through carphone warehouse, tried all sorts of tests even with guys in the shop using different network sim cards and issue continued on all handsets so got a refund to purchase through amazon instead in case of batch issue, which worked fine for few months then issue started, so returned to amazon.
Definitely seems a widespread issue that Sony need to address ASAP. especially as after my refund from amazon I pre ordered xz2 as thought I can't be unlucky like this again, and annoyingly after having xz2 since launch start of this month same issue has started happening over the last week, so I'll be returning again for a replacement. Very infuriating that a phone costing £650 and £700 with the xz2 have such an issue with a basic feature of calling in 2018.
For me I'll be replacing as many as I can through carphone until they won't replace anymore so that Sony will hopefully get their act together.
I encourage anyone with same issue to return theirs for replacement where possible and get the supplier to note the call issue so hopefully Sony can see how big an issue this is, and very annoying this issue seems to be across all recent flagship models at least!
