Facetime audio breakdown at 45 minutes

I recently started using Facetime for audio calls from my Macbook Pro - initially by connection to my tethered iPhone, then by enabling wifi calling. Everything is working as expected, with great sound quality via both FT audio, and phone. Except, at around 45-48 minutes into a call, without fail, the person on the other end begins to get garbled audio from me, including echoes, distortion and glitches. When I hang up and start a new call to the same person, it's all clear again. This has happened on 100% of calls I've made from Facetime, and always at the 45-48 minute mark.

I've checked all the appropriate settings in Facetime, and also the power/sleep settings for my MBP.

Any ideas where I should troubleshoot?

MBP 2.6GHz, 8GB

OS X El Capitan 10.11.5

Facetime 3.0 (2701)