There are two ways you can configure the inbound calls; both start on your PBX side with Ward's template. On the V1VoIP portal side on the "DIDs' -> 'My DIDs' page click on the Forward icon on any DID (it's the right-most icon - mouse-over shows 'Forwarding'.)
1. IP based: Chose 'Forward to 'IP Address/PBX' and in the field below type the IP of your PBX and make the T/O 25-30 seconds. The way my DIDs work is (on the same page at the bottom under 'SIP Type') US DIDs are 11 digits and Canadian ones 10 (that's the only way I get incoming properly routed to the DIDs). No need to alter any other information on your trunk's side.
2. Auth based: On the same page as above, chose 'Register Device/Trunk' and there will be two fields added below with your 'Username' and 'Password'. Copy that information in the following format on your trunk's 'SIP Settings' -> 'Incoming' -> 'Register string':
'Username':'Password'@sip.v1voip.com/'Username' - replace info as described above and remove the quotes. The T/O and 'SIP Type' comments from above apply here too. The rest of the fields on that page remain empty.
NOTE: this approach works with ATA devices too.
If you have many DIDs you can pre-configure the option (IP based or Auth based) on the 'Pre-configuration' page under your name (right-most pull-down on the top of the page) on their portal.
Also, note
@krzykat's comment - while I agree that both those options may work without defining the IPs under 'Termination' -> 'Endpoint Request' it's worth spending the additional seconds to put in those IPs even if dynamic (just change them as required.)
Although the portal is quite basic, the price / options / voip quality ratio is quite attractive.