From Forum Nokia Wiki

A telephone call can be made from a Java ME application by using javax.microedition.midlet.MIDlet.platformRequest().

On some devices the MIDlet has to be closed (for example on Series 40 devices), before the phone call can be placed. The application can take this into account by checking the return value of this method call. The value is true, if the MIDlet suite MUST first exit before the call can be made.

It is also possible to send post-dial DTMF tones along the phone number. However, it is not usually possible to send a DTMF tones sequence to operator service from a MIDlet.

On some devices (on Series 40 side) the length of the dial-string is limited.

The following code is used for making the telephone call

String telNo = "tel:+9682651761543";
platformRequest(telNo );

It should also be noted that user is also provided means to initiate a phone call from MIDP LCDUI PHONENUMBER type TextField or TextBox. User is also able to fetch a number from native phone book (or contacts) application to this type of TextField/TextBox. The Call and Fetch operations are available in Options menu or directly from softkeys depending whether there's also application provided Commands in the same Displayable, or Form Item in the case of TextField.