Go to your trunk, look for “Asterisk Trunk Dial Options” add the ‘r’ option. Be warned though:
Generate a ringing tone for the calling party, passing no audio from the called channel(s) until one answers. Without this option, Asterisk will generate ring tones automatically where it is appropriate to do so; however, “r” will force Asterisk to generate ring tones, even if it is not appropriate. For example, if you used this option to force ringing but the line was busy the user would hear “RING RIBEEP BEEP BEEP” (thank you tzanger), which is potentially confusing and/or unprofessional. However, the option is necessary in a couple of places.