This is caused by someone doing a port scan seeing you have UDP port 5060 (SIP Port) forwarded and then trying all the known asterix/trixbox hacks to do toll fraud.

Try turning on DID error Tone in MMC210.
This will stop the calls ringing.

Better option is to firmware upgrade as the newer versions (>4.7 from memory) have builtin security updates to stop this behavior.

Alternatively you can restrict the UDP 5060 in the router to only accept connections from your sip provider IP address's. Most DSL routers can't do more than 1 ip address (or a range if you're lucky) so this may not be an option.