Yes, to be perfectly honest that CAN happen. Its telecom politics, and thankfully it doesn't happen often.
Simple explanation: It sort of has to do with how Ymax communications (the parent company for MJ) works. Some smaller telecoms don't get alone with them due to pricing conflicts for network hops.
Longer explanation: I could be wrong about some of these details, but this is how I understand it. Ymax owns POTS infrastructure that major telecoms use to route calls. Every network "hop" is charged a fee, but most telecoms make it back when a "hop" is made in the reverse direction (calls both ways). Smaller telecoms that don't have their own infrastructure don't have the opportunity to make back as much since they are spending money on outgoing "hops" but not making it back on incoming, so they raise their fees for incoming connections to stay afloat.
Ymax's whole shtick with MJ (and why it is so cheap) is that they make money on more than just your annual service fee. VOIP doesn't have to pay the same fees that POTS hops do, and therefore they are immune to some outgoing call hop fees, but can still charge them on the way in because the parent company owns infrastructure. I've heard this described as a "roach motel" concept; the money comes in from calls hopping onto the network, but it doesn't have to come out when a call goes in the other direction. This business model seems pretty ingenious to me- we as the customers aren't charged a monthly fee, and the parent company makes enough money to keep improving the service. But the problem is those pesky smaller telecoms that charge higher fees. If they can't agree on a price that fits into the budget for Ymax/MJ's hops, they can just refuse the connection. It will still work in the other direction however, which is why they can call you.
This isn't necessarily unique to Magic Jack as I understand it. ITs telecom politics.