The most common way is to use phone tracker, it use free gps

There are generally 2 ways of tracking a cell phone at present:

Tracking the cell phone’s GPS location:
For this to work, a GPS chip or tracker must be previously installed on the cell phone you are trying to track. Newer cell phones have this already. If you’re not sure if the target phone has such capability, you may also try asking the network provider if they offer such service. They’ll be willing to help if they do. Just be ready for their instructions as they’ll make sure your motives are legitimate and you’re not stalking on someone.
Tracking the cell phone’s call logs and text messages:
There are now programs available that show you the call logs, the SMS messages that a cell phone sends and receives, and many other details.

There are generally 2 ways of tracking a cell phone at present:

Tracking the cell phone’s GPS location:
For this to work, a GPS chip or tracker must be previously installed on the cell phone you are trying to track. Newer cell phones have this already. If you’re not sure if the target phone has such capability, you may also try asking the network provider if they offer such service. They’ll be willing to help if they do. Just be ready for their instructions as they’ll make sure your motives are legitimate and you’re not stalking on someone.
Tracking the cell phone’s call logs and text messages:
There are now programs available that show you the call logs, the SMS messages that a cell phone sends and receives, and many other details.

Or replace this cellphone completely with a new one, new number and match it up with a new girlfriend

i have recently split up with my girlfriend and i have found out she has been using spy software to read my txt messages and listen to my phone calls, is there a way to stop this from happening? any help will be greatly appreciated this is giving me sleepless nights i feel like im being stalked