Record them on the phone

You don’t need some sort of KGB style mini bug in the phone – a simple, small recorder that is ‘voice activated’ to avoid draining the batteries is fine. Check this Pro-Recorder out.

Simply connect it between the phone and the wall mounted socket on the main phone line.

Introducing the worlds first and only NO BATTERY DRAIN recorder (When used with single line phones) a 34-Hour Portable Digital Telephone Recorder with aTime/Date Stamp. Designed and manufactured exclusively for the SPY Industry!

This little baby will do everything you need to catch them in the act – is it oral sex when it’s recorded?

The Pro-Recorder records over 12 hours of phone conversations on a single set of AAA batteries even if it takes weeks or months!

The Recorder automatically turns on when telephone receiver is lifted, and turns off when hung up.

NO BATTERY DRAIN while waiting for next call from any extension. (Single line phones)

Tech. features:

• Automatic “No Battery Drain” Telephone Recording on Single Line Phones
• Universal Cell, Cordless, and Pay Phone Recording interface included. (NEW)
• Time/Date, Length of Call Display for Each Recording Shown of Display During Playback
• USB Digital Interface for Quick File Transfer to Your PC
• Also Can Be Used as a Voice Recorder w/built-in Microphone
• Also Works with Vonage VOIP phones
• Microphone Sensitivity Selection
• Remaining Recording Time Display
• VOR Recording Eliminates Gaps in Recording
• Variable Playback Speed
• Quick Message Search
• 3 Quality Mode Selections
• Recorded Message Monitoring
• Backlit LCD Display
• Optional AC Adaptor

Includes Full Accessory Package:

• External Microphone
• Batteries
• Earphone
• USB Cable
• Voice Manager CD
• Users Manual
• Hand Strap
• Telephone Interface for Single Line Phones
• Universal Cell, Cordless, Digital, Pay Phone Interface
• Size: 35mm X 14mm X 90mm
• Optional AC Adaptor

Get more info here.