[icanhackit]

Calls are supposed to be monitored (usually done manually) to prevent criminal business [...] In higher security levels phones are labor intensive. An officer has to escort [...] from their cell to the phone, and stand there for the duration of the call.

Sounds like part of the solution is right there, at least in higher security prisons, which is to have the escort also monitor the call. Record all calls regardless and perform spot-audits so that any potential collusion between inmates, outsiders and escorts can be eliminated or minimized. A higher percentage of spot-audits should be carried out on repeat offenders and lower percentage on inmates associated with minor crimes - risk can be assessed with a couple of conditional equations. Advise inmates and staff of monitoring system to prevent collusion from forming in the first place.