Melting and phase transitions of phosphorus allotropes have been studied in the 2-11 GPa pressure range using in situ electrical resistivity measurements and synchrotron X-ray diffraction. It was found that the melting curve of rhombohedral phosphorus exhibits positive slope of 58(2) K/GPa and crosses the rhombohedral-simple cubic phase boundary (negative slope of -420(70) K/GPa) at about 8.3 GPa and 1490 K. The melting slope of simple cubic phosphorus can be estimated as +300(90) K/GPa. Calculations of phosphorus phase equilibria have been performed using our experimental and literature data within the framework of the phenomenological thermodynamics. The constructed equilibrium p-T phase diagram accurately describes the experimentally observed phase relations in solid and liquid phosphorus up to 12 GPa and 2000 K.