If after a reasonable period of negotiation, a dispute or impasse exists between the representatives of the public employer and the public employes, the parties may voluntarily submit to mediation but if no agreement is reached between the parties within twenty-one days after negotiations have commenced, but in no event later than one hundred fifty days prior to the “budget submission date,” and mediation has not been utilized by the parties, both parties shall immediately, in writing, call in the service of the Pennsylvania Bureau of Mediation.