Yes. Even if they have a pacemaker implant, they can still use a cell phone. A minimum safety clearance of 20 centimeters between chest and cell phone is recommended to exclude interference with the pacemaker.
You should therefore not carry your cell phone in your breast pocket. However, this is merely an additional safety precaution, since the majority of pacemakers implanted today are resistant to interference from cell phones.
Patients with other electronic implants (e.g., insulin pumps) should consult their physicians about the resistance to interference of such devices.