Utter nonsense
Who does Wallace think he's kidding? What terrorist would use an unencrypted landline to call another terrorist and chat about who, how and when they'll attack? Does he really think we're that stupid?
All terrorist incidents involve mobile - mobile communication, most of it over encrypted messaging services. The Westminster Bridge attacker was on WhatsApp 3 minutes before he drove across the bridge. He wasn't at his mum's house using her landline to correspond with his co-consiprators.
Mobile - mobile communications do not use GSM gateways. It is uneconomical to do so, and do you really think a mobile network will connect their services through a GSM gateway company or through a direct interconnect with another mobile network? Of course not. They all have a well-documented hatred of GSM gateways.
Wallace should be concentrating on regulating encrypted services rather than a 30-year old technology that isn't used in the way he suggests. Unless, of course, the all-powerful mobile networks are influencing Ofcom and the Home Office behind the scenes to protect their commercial interests. But that could never happen here, could it...?