Clarissa Ramon points to the problem:

According to Prison Legal News, the cost of making a long distance phone call from a prison in Oregon includes a $3.95 connection fee plus .69 cents a minute, costing $14.30 for a 15-minute call. Compare this with making a public call outside of prison which costs anywhere from 05 to 10 cents per minute for long distance calls on landlines, costing a maximum of $1.50 for a 15-minute call. For many families with loved ones behind bars, the choice between accepting a collect call and putting food on the table is a real and painful decision. 

The prison phone system in the US is estimated to gross $362 million, with 42% of that, or $152 million, going to the prisons. E.D. Kain sighs:

Maybe the state thinks this is justified. After all, to house a prisoner – even a nonviolent one – costs upwards of $30,000 a year, sometimes more. You could basically pay people minimum wage just to stay out of prison and save money in the process. $15 bucks a call is just the state making up for lost revenue. Of course, it would make more sense to reform our prison system and our drug laws so that fewer not-menaces-to-society ended up behind bars. But again, the powers-that-be and the many groups that stand to lose money from drug reform and prison reform will lobby hard to see the racket continue perpetually.