It is not uncommon for a carrier to play a “number is not in service” or “number has been disconnected” when there is no response to an INVITE. That, per se, is not Momentum’s fault. However, a decent provider has a way to handle calls when the customer’s equipment is down or otherwise inaccessible.
Usually, there are settings on the portal that will direct such calls to a mobile phone or other external number, or play an announcement or perhaps take a voicemail. Possibly, Momentum has this feature but it was never set up, or it was set up and not working properly.
Back to the original problem, do you have settings to keep the NAT association alive e.g. Expiration = 120, Qualify Frequency = 60?
Is there any indication by Asterisk that the NAT association was lost (does “unreachable” appear in the Asterisk log)?
Is there any indication on the Momentum side that registration was lost (do they show registration status on their portal)?
What is the value of SIP Server? If this is a domain name (rather than an IP address), is it resolved with SRV records or A records? If multiple records, are all the servers up at the time of outage?
If all of the above looks good, I suspect a router/firewall issue. Make/model? Any special settings for VoIP? Does it get a public IP address on its WAN interface? If not, why (modem is configured as a gateway, ISP is doing NAT, etc.)?