Yes. The Dexcom transmitter communicates directly with your compatible mobile device, even when it is paired with the t:slim X2 pump.
The Dexcom transmitter can be paired with one medical device and one non-medical device at the same time. (e.g. A transmitter can connect to your t:slim X2 Pump and a mobile device running the Dexcom app, or to a Dexcom receiver and a mobile device running the Dexcom app.)
Important note about Dexcom support:
For "no restarts" alert or physical problems with your sensor or transmitter, such as bleeding, bruising, skin reaction, adhesive issues, applicator issues, or broken wires, you should contact Dexcom using one of the options at dexcom.com/contact.
If you have paired a Dexcom CGM with your Tandem insulin pump, and your question isn't answered in this help section, you should contact Tandem customer technical support for all other issues by calling (877) 801-6901.