tried that. it doesn't do what I am asking for:

1. Always use 2G data network connection

2. When needed (examples)
2.0 When web surfing change the data connection (from 2G to 3.5G)
2.1. When using/starting or receiving a SIP call change the connection (from 2G to 3.5G)
2.2. When downloading or uploading email attachments also change the default 2G connection

3.0 After the above examples/actions end (2.0, 2.1, 2.2, etc) the data network connection reverts to the default 2G when in standby