This should also work for E60/E70/N80

I use inphonex.com VOIP service, it is cheap, interface and feature set is great and never had any issues.

I was looking for a way to set it up on my E61, after a bit of trial, here goes the settings:

Profile: InphoneX
Service profile: IETF
Default access point: Select your wifi or gprs access point
Public user name: sip:your_inphonex_7_digit_account@sip.inphonex.com
Use compression: No
Registration: When Needed
Use security: No

Proxy server:
Proxy server address: sip:sip.inphonex.com
Realm: inphonex.com
User name: your_inphonex_7_digit_account
Password: your_account_password
Allow loose routing: Yes
Transport type: UDP
Port: 5060

Registrar server:
Registrar serv.addr.: sip:sip.inphonex.com
Realm: sip.inphonex.com
User name: your_inphonex_7_digit_account
Password: your_account_password
Transport type: UDP
Port: 5060

Enjoy