Update: HTC has told us that the problem is limited to One M7 (last year's flagship models) with the One M8 unaffected, and has sent a revised statement (below). A fix is rolling out in Australia with 'other regions very soon'.
HTC has confirmed to TechRadar that it has found a fault with its One M7 handsets and is working furiously to fix it.
The issue was highlighted after some users, having downloaded the recent HTC Service Pack from Google Play, found they couldn't make or take calls.
An HTC spokesperson has given us the following statement, confirming the brand is aware of a fault and promised a 'fix will roll out in the coming days':
"We are aware that some HTC One (M7) users have recently been experiencing audio issues making network calls on their devices.
Neither WiFi calling nor other HTC devices are impacted at this time. We're working around the clock with our technical teams and partners to resolve the issue as a number one priority.
We will provide further updates as soon as we can. For anyone experiencing this issue with your HTC One (M7), you may want to try WiFi calling, and we ask that you please contact your local Customer Care: www.htc.com/support/ (opens in new tab)."
In the dark
The exact problem still hasn't been confirmed; some users have speculated that the new Service Pack halts the function of the Google framework, which causes the modem to behave erratically.
Similarly, HTC is only confirming the issue is affecting 2013's HTC One model, although users are reporting that the One M8 is also suffering from the same calling problem.
The issue doesn't seem to be hugely widespread at the moment, but is affecting a significant enough number of users for HTC to issue the statement above. In the meantime, it's worth not accepting any system updates and cancelling the automatic updates to HTC apps in the Google Play store until HTC has resolved the problem.
This unexpected problem with HTC's premium phone is in spite of the fact that the HTC One M8 has received many accolades including TechRadar's nod for best smartphone 2014.
We'll update this story when further information becomes available.