At the end of the episode the car which drives onto the drive has a '51' plate meaning it was registered in late 2001 - early 2002 despite the action taking place in 1999.
A trimphone rings in DC Bethell's home in 1973, but with the wrong sound. Trimphones made an electrical sound, but the soundtrack played the bell ringtone of a more usual phone of the time.
When 1973 Paul goes to answer the phone at 29 minutes, it is ringing like a traditional phone. The phone is a trim phones which rang with a trill sound.