It's very common to find broken antennas on the EXP9300. The original antenna is not available but P1Repair offers a replacement that looks good and works just as well as the original.
The antenna is a metal wire with a rubber housing. Some phones only need the rubber housing but others the wire has also broken off and both pieces need to be replaced.
The following image shows the metal interior part of the antenna found in the EXP9300 cordless phones.