Indian seismologist says that mobile phones tend to fail 100 to 150 minutes before an earthquake. Going by the details the article gives, the mechanism must be the frequency shifts he describes – if transmissions around 1000MHz are shifted up to 1800-1900 and 2000MHz, that would put them right in two out of four GSM wavebands, close enough to the WCDMA/CDMA2K 2100MHz band, and would include the 850 and 900MHz bands in the freqs getting shifted up to interfere with them.
It’s unlikely that preliminary tremors would change much – they might spoil the very finest details of the radio-planning in a fixed cell GSM network, but not by enough to do any damage, and a CDMA system would cell-breathe around it. Point-to-point microwave backhaul links might be affected, but then, those need very tough construction indeed.