I want to increase the antenna strength on my cell phone.

Would soldering a piece of wire to the antenna already on the phone and run the wire inside the case of the phone work?

I was thinking about using 28 gauge wire.

Can the wire be left inside the insulation or should I remove the insulation?

Thanks.

 

Bad idea. If anything so simple would have improved reception, don't you think the engineers would have done this already?

 

In general, antenna systems are carefully designed to match the frequency at which the signals received are expected. In your case, randomly adding wire to your GPS antenna will make the performance worse, not better. Wire inside the phone is likely to pick up electrical noise generated within the phone and make your reception even worse. The GPS antennas are often active antennas anyway, and don't follow any simple rules like "increasing the length of the antenna will improve performance".