On my brand new myTouch 3G, I could not dial my sister's mobile phone.
The problem was directly related to the Contacts app. I could not view any of my sister's information. So I used my laptop to review my sister's data in the Contacts view of Gmail in the Firefox web browser. There, I noticed I had defined a Instant Messenger username with the incorrect type. I corrected it, re-synced my contacts on my phone, and now it works fine.
To confirm that fix, I found another person in my contacts that also had an incorrect type set for a instant messaging username. The acore process crashed for that person as well on my phone. And after I corrected her information in Firefox, and re-synced my phone, then I was able to view the information for that person on my phone; the acore process stopped crashing.
In summary, for Android 1.5, if a person has a instant messenger username set, then the type must also be set and set correctly.