I have seen that issue on my Verizon account. It may be a network/congestion issue.
JUNE 4th: There appears to be an outage with at least one U.S. cellular carrier, with customers unable to place or receive calls. AT&T, T-Mobile, and Verizon customers have been complaining about the issue on social networks, and there are also reports of outages on the Down Detector website.
I have been testing on the Tello $10 plan and have no issues with any delay dialing out. I have tested using WiFi Calling and without.
Samsung Galaxy S9 phone.