Well that is incredibly strange! Do you happen to notice how many bars of signal your phone shows when this happens? Does this tend to happen more frequently when you are in a specific area?
I have to agree with Chris - this is super strange! What type of phone are you using? Does location matter? As Chris asked, when this happens how many bars of signal do you have? Lastly, have you talked with Tech Support regarding this yet?
Thank you for your responses.
This happened at home office in an excellent signal area and on WI-Fi calling. Using a ZTE. Discussed this issue with tech. several times. Finally received an answer that seems to be working. Will get back if not. Apparently, having my Connection preferences setting to Wi-Fi preferred was (maybe) causing this situation. Tech asked me to place setting on Cellular preferred which uses Cellular Network for calls. If Cellular unavailable, use Wi-Fi. Makes sense, I guess, since I am in a very strong signal area. However, my Wi-Fi signal was also strong, so I do not know why that would cause a busy signal for callers. In any event, calls seem to be going to voicemail now when I am unable to answer instead of a busy signal.
On my android:
Go to: Settings>More>Wi-Fi Calling>Cellular preferred
That is good to hear! Please do let us know if the situation changes.