Page 116 of the OBi Device Administration Guide states:
$2 is an internal variable containing the called number of this outbound call, after any digit map transformation in the matched callee object
$2 is not expanded, however, and is passed as the literal '$2' in the following OutboundCallRoute rule:
{(Mpli):pp(3*$2)}
Speed Dial 3 is used and the targeted OBi receives the call, but with a payload of '$2' instead of the called number.