After a huge pain and lot of time on getting GPRS dial-up Internet connection on Mac OS X with Sony Erricsson D750i from T-Mobile, it works!!! The trick is:

1. Download SonyEricsson modem sripts from Ross Barkman’s page.
2. Don’t use pre-defined settings, but create your own data account for Internet access. Of course you can use a configurator from T-Mobile.
3. Use Sony Ericsson GPRS modem script with CID corresponding to newly created data accountD.
4. Put internet.t-mobile.cz instead of phone number.
5. And last but not least … use username and password (gprs/gprs) regardless T-Mobile’s instructions of authentication being not required.