I use at home and at work Asterisk PBX. It is relatively easy to setup and works great. For those of you who have setup one and wondering settings on Nokia E61, here is the settings:

Profile name: Anything you want
Service profile: IETF
Default access point: Select your wifi or gprs access point
Public user name: sip:username@host.domain.com
Use compression: No
Registration: Always on
Use security: No

Proxy server:
Proxy server address: sip:host.domain.com
Realm: asterisk
User name: username
Password: password
Allow loose routing: Yes
Transport type: UDP
Port: 5060

Registrar server:
Registrar serv.addr.: sip:Asterisk
Realm: –
User name: –
Password: –
Transport type: UDP
Port: 5060

Replace your Asterisk username,password and hostname/IP in above and you are good to