Welcome lucm!

I have no such problems with calls on the N900, so my guess is that this is a configuration issue or that your N900 is swapping due to many open apps.

I suggest you start by installing the CSSU (http://wiki.maemo.org/cssu), since it improves the user experience of the phone overall.

Furthermore, with the CSSU you will be able to alter the phone app settings so that it sticks to portrait or landscape mode, if you wish so.