If you want to reach a VoIP.ms subscriber from Asterisk without using the PSTN, there is a way to do so via SIP URIs.

Here's what I added to my /etc/asterisk/extensions.conf:

exten => 1234,1,Set(CALLERID(all)=Francois Marier <5555551234>)
exten => 1234,n,Dial(SIP/sip.voip.ms/5555556789)