One possibility: If you have the $15 USD/year acct. (VoXLite), it only includes 1 SIP Registration. So only one provider can register at a time.
Other possibility: If the provider is MyNetFone...there has been reports of intermittent issues with registrations. MNF is one of the few providers that requires regsitration to make outgoing calls. Furthermore recently users whose MNF accts. were previously registering fine, have experienced problems with registrations (in some cases the registrations drop suddenly).
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