There are a couple of things I can think of off hand that could cause low volume on transmit.

Questions -

Is the phone new or used?
Did it ever work?

If it's new then I would look at the mod you did - maybe you've got something shorted. If it's used, then it could either be a bad transmitter or possibly dirty contacts on the touch tone pad - if it's an older unit.

The easiest fix would be to try to clean the contacts. If you don't have a burnishing tool then you could use a dollar bill (or I guess a pound note [5 is the smallest, no?]). Don't use plain paper - it could damage the contacts. Just run the bill back and forth across the contacts a few times.

If that doesn't work then it could be the transmitter - or even the receiver. You'd have to get some spares and try replacing them.

Sam

"Where are we going and why are we in this hand basket?"