Most cellphone antennas are designed to provide good performance in two frequency bands, one around 900 MHz (a bit higher in Europe, a bit lower in North America, for example) and the other around 1800 MHz (again, Europe band is a bit lower in frequency than the North American band, and then there is the UMTS band in Europe slightly higher than the North America band). So, the impedance matching situation gets kind of complicated. If you decided to modify the antenna by adding some additional wire length, I would say that chances are about 98% that you will make it perform worse on one or the other band, and about 2% that you got lucky and it improves in both.
Its also interesting to note that in the lower band, the one around 900 MHz, the so-called antenna is in fact only half of the radiating body. The other half is the phone housing itself (or the conductors inside it, mainly the ground plane on the main circuit board). So Audioguru is surprisingly correct when he says that the rest of the antenna may be inside the phone. Yes, in fact it IS the phone. This is still true but to a lesser extent at the higher frequency band.
The best advice for getting a better connection with an existing phone is to get an external antenna kit (if available) and use it, or hold the phone further away from your body, or get the phone higher from the ground.