I had troubles with DTMF decoding to, but it was noise, not gain!
I use it in both modes (single-ended and diferential input) and the gains are:
diferential:
R1,R4 = 200k;
R2 = 68k;
R3 = 56k;
R5 = 220k;
single:
Rin=2k2; Rf=82k;
U must put a capacitor between the transformer and the Rin:

|----||---++++--------------- pin2
3 0.1uF Rin | (MT8888)
3 |--++++----- pin3
3 transformer Rf
3
|-GND

I wish this could help u.

[]'s
Evandro Zilio de Souza