Yuck. I agree with Brian, find another provider.
ignore_early_media should work at masking the invalid number announcement but will have a lot of sideaffects - it won't be a nice user experience to hear the wrong ringback (it'll be specific to the destination country, and some people can pick a custom one), mask any IVR announcements the remote side does in early media, and not know they've dialed an invalid number and instead have to wait 20s+ before hangup with no explanation.
Whatever timeout you pick might also have false positives with people taking a while to answer the phone (eg when they're at the wrong end of the house).