Asha 501: VoIP, TLS and certificates

I've recently added sip account to my Asha 501. It works great when I use UDP or TCP transport, but when i switch to TLS it can't connect to the server. I think, this may be caused by self-signed server certificate.
So, is there a way  to add my own certificates to Nokia Asha 501 ?

I am trying to use TLS in voip environment  and I am in a cross roads.
The guy who manages the asterisk server sent me two certificates: One CA.crt and another User.pem.
I have been able to make the asha 311 accept the CA.crt (after changing it to a ca.der file, according to this post:
But when I try to download the User.pem certificate, I get a message saying "no gateway reply".
So, I have two questions. ONe is, do I relly need these two certificates - is it that the asterisk guy is over complicting things?
The other is. if I need this pem file, how can I make the mobile accepct it?
thanks in advance.