Usually you can use your mobile phone from Europe for the first few days, but we advise you to get a local number asap. Just buy a local SIM-card (Chine Telecom or Unicom), which usually comes with a package of minutes. Keep the original business card sized SIM-card holder, since it has the PIN numbers on it, which you might need. Should your card ever get lost (or stolen) you can apply to get a new card with the same number issued if you still have this proof of purchase.
We recommend the friendly guy in front of Wumei mart on the PKU campus (map). He can sell you a student discount package with SIM-card, free minutes and a cheap international plan. Please ask him to handle the trigger call to unlock the SIM-card.
If you have an iPhone for which you need a miniSIM-card then either purchase a mini-SIM or ask them to cut out the chip with a pair of dedicated pliers (they might charge RMB 20-30 for that service.)
You can also enquire about cheap other options, e.g. all incoming calls for free (10¥/month), SMS-packages, cheap international call-by-call numbers (for this just add „12953“ to any foreign number, e.g. „12953-0049-your mom“).
Wumei Minimart 物美超市, 北京市海淀区五四路 (map)