There are a large number of potential sources of hum and each situation will narrow them. The first thing would be to quantify the observation. I am assuming it is power line frequency, although that may not be the case. It is also useful to notice whether it is fairly pure or rich in harmonics, or even if it is predominately the second harmonic, which would be produced by full wave rectification in a power supply.
Classically in telephony, hum is caused by imbalance or things like split pairs that disturb the balancing capability of the twisted pair. However, as has been pointed out, hum can only enter a system at an analog point, and the OP indicates that the only analog point in the system would be inside the SIP phone, unless there is something analog beyond the T1, but that is probably outside of his control. Since Asterisk is connecting a T1 to a SIP phone, there is no way anything inside the box (PC) can be the cause of the hum. If it had FXO or FXS cards in it, magnetic coupling or even electrostatic coupling inside the PC would definitely be a consideration, and would most likely manifest itself as some sort of buzz that was NOT related to the power line frequency. The comment about the wall transformer on the SIP phone is very germane. If it has a bad or inadequate filter capacitor that could allow hum into the SIP phone through the power voltage. It would most likely be 120 HZ (US, 100 Hz in many other countries) and would have additional harmonics, because it isn't sinusoidal. One test would be to power the phone from a known good laboratory type power supply as a test. The other main culprit in this case would be coupling of magnetic fields into the phone itself, either the electronics inside the housing, or the handset components, or even the handset cord. Proximity to any electrically powered device with a large transformer would be a potential source. Even a nearby CRT terminal or monitor. An easy test for this would be to move the phone and/or handset. Generally just rotating it 90 degrees will make the loudness of the hum change noticeably. If that is the case, then identify the offending source and move or replace it. This situation is actually simplified because the SIP phone is the only thing that could produce the symptom. Its just a matter of determining how it is entering the phone, through the power supply or directly into the electronics from a nearby source. Wilton
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