any EMF radiation can affect the line speed or block the signal. Some common disturbers:

1. One or more cordless phones.

2. Nearby radio stations or Ham radio equipment.

3. Garage door openers near the data line.

4. Bridge Tap (which means you have a short circuit on the line somewhere)

5. Sodium lights or other special lights near the line.

6. Bad wiring or filters on the line.

7. Crosstalk if next to another line or wire. How many twists per foot is the wires getting?

8. Power lines near the data line.

9. other electric appliance near the data line.

10. When all else fails, try to rewire it. Check the jacks, the connectors, etc.

We learned this from DSL school, almost anything can interrupt the line or slow down the data. A line coil on the line, a bridge tap, or a disturber can knock out DSL or slow it down to an unacceptable level. I assume that T1 works the same but doesn't use the phone line?