Does N900 have some firewall that could block the outgoing SIP calls voice and is there something I can do about it?
When I'm calling from SIP account (my operator Saunalahti Finland provides a service called Nettipuhelin), I can hear the other person just fine but he/she can't hear me. So could this be some proxy, firewall etc. related issue or what?

I'm using 3G and following settings:

Transport: UDP
Outbound proxy: voip.saunalahti.fi
Port: 5060
Discover public address: checked
Loose routing: checked
Keep-alive mechanism: Auto
Keep-alive period: Auto
STUN server: stun.saunalahti.fi
STUN port: 3478

I've also tried to leave STUN-settings empty, because Saunalahti recommended to do so (some possible NAT problems), but it didn't help.