If the interference is coming into your telephone via the RJ-11 cable, then the type of ferrite material you use to suppress the interference will matter. Engineers refer to the type of ferrite as the "mix", meaning what type of ferrite material is mixed to make up the device. These mixes are simply given numbers that are common place in the industry.
There are two mixes that will help knock down interference from ham radio and CB transmissions: type 75 and type 31. The type 75 material is excellent at suppressing frequencies in the 150 kHz to 10 MHz range. In ham speak, this would cover the 160 meter through 30 meter bands. The type 31 material provides broad coverage from 1 MHz to 300 MHz. This would include CB and the 160 meter to 1.4 meter ham bands.
Ferrite materials comes in a variety of physical shapes but the two that are most usable for your application are the toroid (doughnut looking) and the split sleeve type. Either type will work.
When installing the ferrite, plan on several turns through the core so select a core with a large enough hole for this purpose. The reason is that, in general, the effectivity of the choking action goes up as the number of turns squared. So for example, 4 turns is 16 times more effective than 1 turn. Space out the turns evenly on the core, wrap them tightly around the core, and do not overlap turns.
If you are not certain which core material to use, both can be applied to the same cord. Just place one after the other, winding each one individually. Alternatively, talk with your ham neighbor. Most hams are more than willing to help a friendly neighbor with any interference problems. At the very least, your ham neighbor can let you know what bands he or she is operating on so that you can select the best ferrite mix for the situation.