I´m afraid that feedback is an inherent effect to analog telephone lines. It is because of the hybrid transformer used to convert the 2 wire side of the line to the 4 wire side. Most of pro hybrid adapters can be adjusted to avoid this effect, but this equipment is quite expensive. We have some of equipment of this kind here that haven´t been used since we started working with ISDN lines, which leads us to the digital era.
Basic digital telephone lines work at 64kbps, being sample rate equal to 8kHz and wordlength equal to 8 bit. Codification follows A-Law in Europe and mu-Law in other countries, contrary to our common linear PCM files. These are said to follow the G.711 protocol, wich we use for incoming calls in breaking news or intercom in OB vans. An improved method is used when quality is an issue that is called G.722.
You won´t find any problem to encode your recordings to with this specifications, at least for the first one. Some PC based apps will do it.
Or give you A-Law or mu-Law options when going to export a sound file.
You may try to apply a low-pass filter with cutoff freq at 4kHz too, it´s worth the shot.
If not, you may give Speakerphone a try.