Electronic circuits, devices for phones

1. Cellular Phone calling Detector
2. Phone line indicator
3. Phone "Hold" With Music
4. Soft Musical Telephone Ringer
5. Telephone amplifier
6. Cut Phone Line Detector
7. Telephone line monitor
8. Phone Busy Indicator
9. Phone In Use Indicator
10. Phone In Use Indicator-2
11. Ringing Phone Light Flasher
12. Simple Phone Tap
13. Telephone Hold Button
14. Telephone recorder
15. Telephone Record Control
16. The Link Telephone Intercom – Every Home Should Have One!
17. The Link A2B+1 (the Link Telephone Intercom - DTMF version)
18. The Link 4+0 – Internal Intercom
19. The Original 2 Phone Link Design

Other:

1. Intercommunication (Intercom)
2. Intercom
3. High Quality Intercom
4. Cellular Phone calling Detector
5. Isolated Telephone Interface
6. Voice Scrambler
7. Voice Record / Playback Circuit
8. Simple speakerphone