Never got a response on my bug ticket, but it appears this issue is fixed in 16.5. After installing it I no longer have this problem, and I have two coworkers who were affected who report the same.

I just started receiving reports of users of my app having this problem (phone extension not working) but I could not replicate it when I was on 16.1.1. I upgraded to 16.4.1 and now it seems like the recipient is never receiving the digits after the comma even though I can hear them being dialed. If I dial the extension manually it works fine, speed and timing of dialing the digits doesn't seem to be a factor either. I am on 16.4.1(a), on xfinity with an iphone se 2. I have a coworker who is also on 16.4.1(a), with an iphone 14 pro on tmobile who is not experiencing this problem. I did file a bug for this