FVC-70 with SIP trunk - Analog ports use up sessions or become unusable

[OVERKILL]

In one of our buildings we have multiple SIP trunks through a major Canadian provider. The company that does our data wiring and has also historically handled analog phone systems (Nortel) migrated to Fortinet products a few years back to remain "current" and has installed a number of the now older FVC-40, FVC-70...etc series devices, all using analog lines, in various buildings around town. No issues with this type of install. 

However, when we tried to use these devices with the SIP trunks, we've run into a few interesting issues:

1. When you make an outgoing call through an analog port it will call back that number if you disconnect the call too quickly. Say for example you call your cell phone to confirm you have functionality and then hang-up. It will call back the cell about 30 seconds later but there is no connection to the handset/phone you originally made the call from. If you answer it you get dead air and it never re-rings on the outgoing handset. 

2. When you use the analog ports frequently it seems to tie up SIP sessions, suspending them in limbo. This eventually means you get failed calls and busy signals because the device is no longer able to grab a session for the IP sets. A reboot resolves this temporarily.

3. Sometimes instead of #2, you get "the destination is unavailable" followed by a hang-up or just a dial tone again when you try and make an outgoing call through an analog port. The IP sets still work perfectly when this is happening.

The workaround currently in use for the one system is to leave the fax with an analog provider and use the SIP service only for the IP handsets. This appears to be problem-free so far. However, this also negates the cost savings benefit of SIP, as the SIP sessions are a small fraction of the cost of an analog line. 

An older Cisco phone system at another building with the same service works flawlessly for both analog and IP, so I know it isn't an issue with the service. The issues are identical on multiple FVC-70's on different trunks. 

I know this is now an older device and has been replaced by the new web managed version, so I don't know if we should push to have one of those swapped in to see if it resolves the issue or work on Fortinet for an updated firmware that fixes the issue.