One, an outside line (a pair of copper-axial cables) can only accommodate one call at the same time, and digital trunking varies from 10 to 30 depending on the carrier's standards.
Second, first check whether all the outside line are OK.
Third, the OK outside line into the switch's outside port one by one test to see whether the switch's outside port is OK.
Fourth, the extension to call the switchboard phone to see whether there will be a failure to call in the case of the switchboard phone to test the switchboard phone is OK.
The extension of the wiring signal is not good: the outside signal attenuation, the inside signal decay, the inside signal decay. attenuation, internal signal attenuation
One, the external line directly into the telephone to see if there is a murmur, no external line OK.
Two, the internal line if the wireless extension, look at the wireless transmission standards and transmission distance, most of the 30M or so in the circumference of the square, without blocking.
If the inner line is a wired extension, see if there is poor contact.
The above test is best done by IT to operate.