Voip.ms, like many other VSPs, does not provide a free DID, you get a SIP address for the main account and one SIP address for each sub account. I'm not sure, but at voip.ms you may have to configure a sub account in order to have a SIP address reachable from outside the voip.ms network. Once you have a SIP address you can forward your third party DID (such as IPkall) to your SIP address.