RFC5031 is not supported by 46Labs which can explain failing E911 calls. The invite on a URN 911 call contains no "TO" number because the carrier is expected to use the originating callers location information to look up the number of the nearest public-safety answering point (PSAP). These database and mapping servers to accomplish this are not available to 46Labs. This issue is specific to nomad calling from mobile phones so the responsibility to look this information up should lie with the mobile service provider.