Can an iPhone warm up if it has low cellular signal?

It depends on what you are doing. If you are doing things that are processor intensive, such as watching video, or doing data with a low signal, the phone will heat. It will also heat in long phone calls, and FaceTime. As the processor gets used more, it heats up, just like on your computer. However, your computer has a fan or heat sink to dissipate the heat. The iPhone does not have that, it uses the outer case of the phone to dissipate the heat. And, if you have a case on the phone, it will help hold in the heat somewhat.

If you mean with the iPhone get a little warm when trying to use it and you have a limited signal, the answer is yes.