is it possible to send an MMS containing an URL in the text part or in other parts? which mime type I should use?
In this way the user who receives the mms could click on the URL in order to open a WML page.
Some MMS clients can recognize URLs (as well as phone numbers and e-mail addresses inside a MMS or SMS text) and will allow the user to perform actions on the chosen URL. This is dependent on the phone model.
For example, with the Nokia 7560 or Nokia 3650: While viewing the MMS or SMS message, select "Options" --> "Find" --> choose "Phone number", "E-mail address", or "URL address". If any matching text is found, user can highlight their choice and selecting "Options" again will give a menu of actions that can be performed on that URL (or phone number or e-mail address)... including opening the URL in the WAP browser.
You can simply include a URL in standard format within the text of your MMS message. No extra MIME types are needed if you do it this way.