TP5088
DTMF encoder (NatSemi).
When /SNGL is low, only the upper or lower tone (selected by GRP) is given.
OUT is open emitter, connect load to GND.

      +----------+
  VCC |1  +--+ 14| OUT
   LE |2       13|
/SNGL |3       12| D3
  GRP |4  5088 11| D2
  GND |5       10| D1
   X1 |6        9| D0
   X0 |7        8| MUTE
      +----------+

[This information is part of the GIICM]