This shrubby climber is an Allamanda hybrid or selection of unknown origin with large ( 4”-6”) deep purple flowers.  There is a more common selection known as Allamanda "Cherries Jubilee" which has lighter purple flowers with yellow in the throat. Allamandas come from tropical South America.

 Allamanda blanchetii (A. violacea is an old synonym of this species) flowers are similar in color to "Cherries Jubilee" but they are about 2/3 the size of the hybrid. It has long been thought that A. "Cherries Jubilee" might be a hybrid of A. blanchetii and A. cathartica though no one has been able to remake the cross.

 Allamanda is in the Apocynaceae family which includes Plumeria, Carissa, Ochrosia, Adenium, Mandevilla, Nerium oleander, Thevetia and others. All plants with milky sap like these (also typical of the Euphorbia family) are potentially toxic - though a few, like Carissa, are not.  You can identify this family easily by the trumpet-shaped flowers that have radially symmetrical lobes that are usually twisted like little propeller blades as well as the milky sap. Allamandas have smooth margined leaves in opposite pairs or whorls.