I think calling should be enough. I sent a letter and hand delivered it (I am very close to IRCC offices).
I don't think it objectively "delays the process". That really depends on how long you are away. If you're gone for 3 months, obviously that might mean something is being held back. I was gone for 3 weeks over the holidays, let them know via letter. My test was scheduled for 3 days after my return to Canada, I passed, and I already have my oath date (March 8). Full process was 9 months altogether, and I also received and completed a FP request.
Just let them know and hope they will be able to work around your dates