The default for an OBi110 is:
Physical Interfaces > PHONE Port > OutboundCallRoute:
{([1-9]x?*(Mpli)):pp},{(<#:>|911):li},{**0:aa},{***:aa2},{(<**1:>(Msp1)):sp1},{(<**2:>(Msp2)):sp2},{(<**8:>(Mli)):li},{(<**9:>(Mpp)):pp},{(Mpli):pli}
This rule {(<#:>|911):li} makes 911 a special case and sends 911 directly to the PSTN line port with no processing through digit maps etc.
During set up of a new voip provider, if you check the "Use This Service for Emergency 911 Calls" box, then the OBiTALK portal will insert a new 911 rule at the beginning of the Phone Port OutboundCallRoute which will be used in preference to the default as the rules are processed from left to right.
OBi users living outside of North America should be extra careful as their emergency number may not be 911.