Add twilio softphone with integrated AI assistant

backend/src/voice/dto/initiate-call.dto.ts

@@ -0,0 +1,10 @@
+import { IsString, IsNotEmpty, Matches } from 'class-validator';
+
+export class InitiateCallDto {
+  @IsString()
+  @IsNotEmpty()
+  @Matches(/^\+?[1-9]\d{1,14}$/, {
+    message: 'Invalid phone number format (use E.164 format)',
+  })
+  toNumber: string;
+}