honor explicit user action: disconnect from voip account
Calls 43.0 on Librem5 running PureOS Byzantium
When a VoIP account is disabled by setting the slider in the "VoIP Accounts" menu to the left (disable) it re-enables itself after a while (as confirmed by @devrtz when the default route changes).
If the user explicitly sets the account to disabled it should stay disabled until manually enabled again. If the account gets disconnected in enabled state (e.g. because of disconnect from the network containing the voip server) the "Automatically Connect" setting should make the account re-connect when the voip server becomes reachable again (e.g. because the phone reconnected to it the appropriate network).