I've been using fMMS for several months (via T-Mobile USA, havoc mode), mainly just to catch oversized SMS messages that T-Mobile USA seems to convert to MMS.

After updating to fMMS 1.3.0, I started to experience problems sending and receiving SMS (not MMS), phone calls dropping, and general unpredictable/unreliable telephony behavior on the n900.

With fMMS 1.3.0, I've found myself rebooting the n900 occasionally, just to make sure I hadn't inadvertently missed SMS messages or phone calls, when I wonder if my phone is unusually quiet (no texts or phone calls for a while).

I've ultimately decided to just uninstall fMMS 1.3.0 for the time being, to at least regain stability for text/SMS messages and regular cell phone behavior.

Again, I only started experiencing this issues since I updated fMMS to 1.3.0, and I don't recall having these issues with any previous releases of fMMS.

Edit: I reset all settings, reinstalled fMMS, configured for T-Mobile USA again per MMS wiki, and things seem to be stable again.