IMS: Allow DTMF Pause-Dialing

Adding a check to correctly identify and
differentiate MMI codes from pause-dial
DTMF dial strings. This check is based on
3GPP TS 22.030 6.5.2.
Lack of this check was causing dial strings
to be incorrectly identified as USSD codes,
to be sent over IMS UT.

Change-Id: I7e5cae31580ebff76b84b59ca9795cae06171821