I have a Spectrum business voice line. There is a voip modem that provides a simulated CO copper line. On the other side of this copper line is an Asterisk phone system,with 2 lines. One is a real AT&T CO line, the other the Spectrum voice line. This has been working fine since about 2017. Just in the last 2 weeks, callers to the Spectrum line often get a busy signal, but it DOES ring on a backup phone I have on the copper line, and then when I answer it on the Asterisk extension, I just get a dial tone. I have called Spectrum, and they rebooted the modem, but it still happens. Does anybody have some idea how to debug this?