Firewalls that do not use a Consistent NAT can block incoming calls. In this situation the firewall changes the address/port to which babyTEL needs to send the incoming call. If a call arrives at that moment, before babyTEL is notified of the new address/port, then that call will not reach your device. To avoid this, change your firewall setting to enable Consistent NAT.
SIP-aware firewalls can prevent calls from reaching your device. If this is the situation, disable the SIP awareness option on the firewall.
In the case of very strict firewalls, such as the ones allowing traffic only on limited well-known IP ports, your incoming calls may be blocked. Add a rule to allow incoming UDP packets from nat.babytel.ca to be redirected to your device.
Check out our article with configuration recommendations here.