I understand that they won’t want to integrate into the SIP Broker service, but they don’t provide a SIP address. A SIP address will allow other people to call the customer for free on their end as long as they can call a SIP address. And yes, I know, they aren’t likely using the open standard SIP? protocol, but having a SIP address can’t really be that hard to use.
So what should they do? Offer a SIP address. It could be something like sip:telephone-number@ooma.com. If they don’t want to use Ooma.com as the SIP address server, then acquire Ooma.net, and use that. This won’t solve the problem on making SIP calls, but it will at least give that incoming channel that a person can use.