No, I get that. I think Gomez was simply laying out other options. Depending on the business type, it may be easier to change the number and let the contract sit until it expires. I think you may be over complicating it a bit.
AT&T Contract - Not getting out. That's an understood fee, regardless of how he proceeds. It therefore is unnecessary to consider it in the costs of any options moving forward, as it would be identical in both.
Setup PBX to take calls on existing lines:
PC to run PIAF - $300
Card - $500
Incidentals - $150 - $300 (Not excessive, 150W * 24 hours *365 days * $0.108 per kWh = $150 per year, plus other periphery cost)
TOTAL: $950 (base)
His point is that if you could go with a VoIP Provider that costed less per year, it may be a better idea to do so. I have no idea what the call volume is, so I can't say what makes more sense, but I can say that the hardware investment would be comparatively negligible, and possibly better in the long run. Sure, you could argue that some of his investment cost could be made up at the end of the contract by selling equipment that he no longer needs, and that may well be a better way to go. It's just a lot of money to throw at a device that is going to be in service for one year, doing the job a $45 device can do, when ultimately he's going to be on a setup that the $45 device can be adequate for once the contract lapses.
If it was going to cost $500 a year with a VoIP provider and $45 in hardware (plus, if you want to be technical, maybe $20 a year for power to a BBB), he's literally saving $350 the first year, and doesn't have to deal with any of the hassle of porting numbers later, reselling used hardware he no longer needs, and just gets to 'one and done' the whole thing. If that were the case, I don't see why he shouldn't abandon the lines he's not using. Or, maybe get one Obi for the main office line and tie that in for the short term (until everyone learns about the new numbers).