Ok I got the WTAI function save to phonebook working on the Nokia 3650.
The error i was getting was caused by spaces in the name I was trying to save.
I.e when creating a link with the attached wtai function like this: <a href="wtai://wp/ap;555600700;JohnDoe">Save to phonebook</a> The Nokia phone activates the save to phonebook functionality without problems.
But if you create the link like this <a href="wtai://wp/ap;555600700;John Doe">Save to phonebook</a> (notice the space between John and Doe) then the wtai function failes.
The XHTML I'm using is created via XSLT and disabling output escaping doesn't seem to help. Substitutting spaces with "& n b s p ;" is not the solution in case anybody thinks of this, since the last ";" makes the wtai link end here i.e John& n b s p;Doe will try to save the name John& n b s p; in the phonebook.
If anybody had this type of problem, and found a solution for it please let me know.