Only one multimediaservicecenter is allowed active at one time in phone. It does not care for anything else concerned about retrieval mmsc. So if you send notification with different url that one in your phone , it wont work. I wonder if I remember correctly, but it is mentioned somewhere in wap specs, one sentence that says something about location to retrieve.
Check from your phone menu-tools-settings-connection-accesspoints-[your provider subject, probably "something MMS"]. here is some interesting info, that you need to change if you want to receive/send messages to different mmsc (for example your own )
After changing you have to take new settings to use: messaging-push options-settings-multimedia message-preferred connection.
If you did not understand, read more specs...
Have fun, I found it interesting to study.