Does anyone know of a reliable SIP iphone app they can recommend?

If you want it to be secure then the only answer is the Toshiba UCedge but that will require the IPedge and NOT the CIX

If you want a generic one the Media5 fone is sold by Media5 Corp who also sell Mediatrix Gateways etc

I agree with carlos, the media5 phone works good for me on the cix,ipedge,and vipedge as well..

make sure you put a good password in it! or it will get hacked!