The plea of the arbitration agreement is procedural in nature and raises a question of jurisdiction that is not mandatory, since it is based solely on the will of the parties, who are free to choose whether or not to refer the dispute to arbitrators.
Where the ordinary court declares its lack of jurisdiction in favour of the arbitrators, the proceedings may be resumed before the arbitrators within the time limit set, with preservation of the procedural and substantive effects of the claim originally brought.