You are correct in stating that providers such as Sipgate offer a
"free" number when you sign up with them, but that number can only
be terminated at your Sipgate account, not any other, so it means you
have to remain with a Sipgate account in order to keep the same number,
basically, leave sipgate and leave the number.

With the service we offer, you're able to choose which provider you
wish to use, and you can terminate the number to any VoIP Destination,
thus not tied to one provider in order to keep the inbound number. So
for example if you have a "free" number from the likes of Sipgate
or any other provider, if that ITSP goes down you would be unable to
receive any inbound calls on your PSTN number.

Example: You have a "free" SIP account and PSTN number from
"Snake Oil VoIP" and that ITSP goes down, you'd not be able to
receive any calls on your inbound PSTN number because the provider is
down. But if you had a number from us that was terminating at your sip
address with "Snake Oil VoIP" you are able to login at our control
panel and change the destination instantly (as changes are real time)
of the number to another VoIP account to ensure your able to receive
calls.