If Telus is gms based you can search google for "gms codes" and look for an option to change the ring time; it is available.
If Telus is cdma based, you may not be able to change it at the handset and need to call the service provider to see if they can do it for you.
I guess the easiest way to tell the difference is if the phone has a sim card. I know Nextel was using sim cards too but there a good chance it is gms if it has a sim card.
Edit - Forget the above - I just looked up the 7520 via google and it says it is a Nextel iDen (?) phone. I googled and came up with this: Posts from the 7520 Category at BBHub
"On the Yahoo! BlackBerry Users Group, Member Candace Shine started a thread wanting to know how to change the number of rings when a call comes in to any of the BlackBerry 7520s deployed in her office.
"Right now it only rings 3 times and then goes to voice mail," she writes. "I have looked at 'Options' under the cell icon and 'Profiles' but do not see where we can change this."
It was then that fellow user Bill steered Candace in the right direction.
"Call customer service...they can change the ring time," he suggests. "They default to 3 rings, but I had mine changed to 6 rings then voicemail picks up."
Candace took a (bad pun alert) "Shine" to Bill's advice.
"Called and got the ring numbers changed!," Candace writes. "They call it 'extended ringing' and it is a time thing not a ring thing. So basically you get 15 seconds of ringing which equates to 6-7 rings. And there is no charge - bonus!"