I am not exactly sure what you mean by "bad synchronization of the RTP protocol" as Mitel phone use a proprietary MiNET protocol for signalling and semi-standard RTP streams, and RTP is just UDP packets so they are best effort anyway.
This can often be caused by a firmware mismatch between the phones and MiVB... Make sure the phones are getting a proper DHCP offer with call server and TFTP defined, as well as appropriate COS and prioritization and that your switch setup will honor that, or if you are manually pointing them to the controller that you are either using Teleworker mode or are defining the IP address for both the ICP -AND- TFTP Server in the phone.
Assert errors are usually an indication of either a hardware fault, or a incompatible instruction, rarely a networking issue. Assert errors are very similar to kernel panics or "blue screen of death" errors, they are caused by an error that the OS cannot recover from in a reasonable amount of time.
Honestly, the first thing I would look at is to make sure the firmware matches the MiVB software... looking at the "version" number can be deceiving, so the best way to make sure is to get a phone to work with proper DHCP for callserver and TFTP and erase the main load and let it pull the main software directly from the 3300.
If you gave us more information on your setup, like what system version, switches, VLAN's, how the phones are homed (you said there is no DHCP option), and any other relevant information we might be able to give you more direction.