Once a year I do some calculations, to figure out which is cheaper for me: a data-only mobile contract and Skype; or a voice contract.
Skype claims that a skype-to-skype call is at 3MB/min, while a skype-to-landline/mobile is at 1MB/min. Considering that most of the calls that are make are to people that don't have skype, I'll use the latter figure.
I sometimes use up to 400 minutes, or less (no carry-over, which is illegal, but we are too scared to fight this). So 350 minutes equates to 350MB skype-to-skype calls. Let's call it 400MB including presence, contact information. I use around 350MB/month when away from my wifi networks. The most applicable data bundle is 1GB, at ZAR290. An "unlimited" package goes for 10x this amount. So, I'll be paying around ZAR320/month for the contract, including their lovely add-ons they always manage to squeeze in.
Next is the cost of skype. If i choose the pay-per-minute option, I'll be paying ZAR0.177/minute, or roughly ZAR61.95/month. This brings my total monthly bill to ZAR381.95, rounding up for the unexpected - ZAR400/month.
I pay ZAR350 for 350 minutes per month, plus a few other items (promotion fee (WTF is that???), itemized statement). The cost of the phone is included in all of that. Assuming I don't go over any of my bundles (350 minutes, 100 sms, 350 GB), then I pay ZAR637/month. And yes, I do often go over my bundles.
So, a data-only contract (it's not data only, but rather the only bundle is data. Voice calls are paid at full rates - ZAR2.85) is 75% of a voice contract. Plus, i pay the same for local calls as I do for international calls when done through skype. ZAR400 vs ZAR637 is a massive difference, making it well-worth it. If I choose the skype route, I end up paying my mobile operator ~ZAR340/month instead of ~ZAR640/month. They will lose out ZAR300/month - or ZAR7,000 over a 24 month contract period.
If I was a mobile operator, I would hate to lose half of my income from a number of my users. But, that's what they get for bending us over the coals, pulling down our pants, and...well...this is a family show. I've used the most expensive skype option, so the prices for monthly or annual subscriptions would only drop.
The only issue is that of battery consumption of skype, and coverage. I spend 99.9% of my time in urban areas with semi-decent data coverage. For the times when I'm in rural/low coverage areas, then I would pay out my of ass to do voice calls. But I'm not too worried about that.
So my point is, yes mobile operators are losing money to VOIP. Yes, they can do something about that. But, they have been dropping their prices in recent years due to consumer demand and dissatisfaction, so they are losing "potential" revenue regardless. I say stick it to "the man" until we collapse the economy with all these work-arounds.